vimdiff: add tool documentation

Running 'git {merge,diff}tool --tool-help' now also prints usage
information about the vimdiff tool (and its variants) instead of just
its name.

Two new functions ('diff_cmd_help()' and 'merge_cmd_help()') have been
added to the set of functions that each merge tool (ie. scripts found
inside "mergetools/") can overwrite to provided tool specific
information.

Right now, only 'mergetools/vimdiff' implements these functions, but
other tools are encouraged to do so in the future, specially if they
take configuration options not explained anywhere else (as it is the
case with the 'vimdiff' tool and the new 'layout' option)

Note that the function 'show_tool_names', used in the implementation of
'git mergetool --tool-help', is also used in Documentation/Makefile to
generate the list of allowed values for the configuration variables
'{diff,merge}.{gui,}tool'. Adjust the rule so its output is an Asciidoc
"description list" instead of a plain list, with the tool name as the
item and the newly added tool description as the description.

In addition, a section has been added to
"Documentation/git-mergetool.txt" to explain the new "layout"
configuration option with examples.

Helped-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 194 insertions, 0 deletions

diff --git a/Documentation/mergetools/vimdiff.txt b/Documentation/mergetools/vimdiff.txt
new file mode 100644
index 0000000000..2d631e9b1f
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+++ b/Documentation/mergetools/vimdiff.txt
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+Description
+^^^^^^^^^^^
+
+When specifying `--tool=vimdiff` in `git mergetool` Git will open Vim with a 4
+windows layout distributed in the following way:
+....
+------------------------------------------
+|             |           |              |
+|   LOCAL     |   BASE    |   REMOTE     |
+|             |           |              |
+------------------------------------------
+|                                        |
+|                MERGED                  |
+|                                        |
+------------------------------------------
+....
+`LOCAL`, `BASE` and `REMOTE` are read-only buffers showing the contents of the
+conflicting file in specific commits ("commit you are merging into", "common
+ancestor commit" and "commit you are merging from" respectively)
+
+`MERGED` is a writable buffer where you have to resolve the conflicts (using the
+other read-only buffers as a reference). Once you are done, save and exit Vim as
+usual (`:wq`) or, if you want to abort, exit using `:cq`.
+
+Layout configuration
+^^^^^^^^^^^^^^^^^^^^
+
+You can change the windows layout used by Vim by setting configuration variable
+`mergetool.vimdiff.layout` which accepts a string where the following separators
+have special meaning:
+
+  - `+` is used to "open a new tab"
+  - `,` is used to "open a new vertical split"
+  - `/` is used to "open a new horizontal split"
+  - `@` is used to indicate which is the file containing the final version after
+    solving the conflicts. If not present, `MERGED` will be used by default.
+
+The precedence of the operators is this one (you can use parentheses to change
+it):
+
+    `@` > `+` > `/` > `,`
+
+Let's see some examples to understand how it works:
+
+* `layout = "(LOCAL,BASE,REMOTE)/MERGED"`
++
+--
+This is exactly the same as the default layout we have already seen.
+
+Note that `/` has precedence over `,` and thus the parenthesis are not
+needed in this case. The next layout definition is equivalent:
+
+    layout = "LOCAL,BASE,REMOTE / MERGED"
+--
+* `layout = "LOCAL,MERGED,REMOTE"`
++
+--
+If, for some reason, we are not interested in the `BASE` buffer.
+....
+------------------------------------------
+|             |           |              |
+|             |           |              |
+|   LOCAL     |   MERGED  |   REMOTE     |
+|             |           |              |
+|             |           |              |
+------------------------------------------
+....
+--
+* `layout = "MERGED"`
++
+--
+Only the `MERGED` buffer will be shown. Note, however, that all the other
+ones are still loaded in vim, and you can access them with the "buffers"
+command.
+....
+------------------------------------------
+|                                        |
+|                                        |
+|                 MERGED                 |
+|                                        |
+|                                        |
+------------------------------------------
+....
+--
+* `layout = "@LOCAL,REMOTE"`
++
+--
+When `MERGED` is not present in the layout, you must "mark" one of the
+buffers with an asterisk. That will become the buffer you need to edit and
+save after resolving the conflicts.
+....
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     LOCAL         |    REMOTE          |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+--
+* `layout = "LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE"`
++
+--
+Three tabs will open: the first one is a copy of the default layout, while
+the other two only show the differences between (`BASE` and `LOCAL`) and
+(`BASE` and `REMOTE`) respectively.
+....
+------------------------------------------
+| <TAB #1> |  TAB #2  |  TAB #3  |       |
+------------------------------------------
+|             |           |              |
+|   LOCAL     |   BASE    |   REMOTE     |
+|             |           |              |
+------------------------------------------
+|                                        |
+|                MERGED                  |
+|                                        |
+------------------------------------------
+....
+....
+------------------------------------------
+|  TAB #1  | <TAB #2> |  TAB #3  |       |
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     BASE          |    LOCAL           |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+....
+------------------------------------------
+|  TAB #1  |  TAB #2  | <TAB #3> |       |
+------------------------------------------
+|                   |                    |
+|                   |                    |
+|                   |                    |
+|     BASE          |    REMOTE          |
+|                   |                    |
+|                   |                    |
+|                   |                    |
+------------------------------------------
+....
+--
+* `layout = "LOCAL,BASE,REMOTE / MERGED + BASE,LOCAL + BASE,REMOTE + (LOCAL/BASE/REMOTE),MERGED"`
++
+--
+Same as the previous example, but adds a fourth tab with the same
+information as the first tab, with a different layout.
+....
+---------------------------------------------
+|  TAB #1  |  TAB #2  |  TAB #3  | <TAB #4> |
+---------------------------------------------
+|       LOCAL         |                     |
+|---------------------|                     |
+|       BASE          |        MERGED       |
+|---------------------|                     |
+|       REMOTE        |                     |
+---------------------------------------------
+....
+Note how in the third tab definition we need to use parenthesis to make `,`
+have precedence over `/`.
+--
+
+Variants
+^^^^^^^^
+
+Instead of `--tool=vimdiff`, you can also use one of these other variants:
+
+  * `--tool=gvimdiff`, to open gVim instead of Vim.
+
+  * `--tool=nvimdiff`, to open Neovim instead of Vim.
+
+When using these variants, in order to specify a custom layout you will have to
+set configuration variables `mergetool.gvimdiff.layout` and
+`mergetool.nvimdiff.layout` instead of `mergetool.vimdiff.layout`
+
+In addition, for backwards compatibility with previous Git versions, you can
+also append `1`, `2` or `3` to either `vimdiff` or any of the variants (ex:
+`vimdiff3`, `nvimdiff1`, etc...) to use a predefined layout.
+In other words, using `--tool=[g,n,]vimdiffx` is the same as using
+`--tool=[g,n,]vimdiff` and setting configuration variable
+`mergetool.[g,n,]vimdiff.layout` to...
+
+  * `x=1`: `"@LOCAL, REMOTE"`
+  * `x=2`: `"LOCAL, MERGED, REMOTE"`
+  * `x=3`: `"MERGED"`
+
+Example: using `--tool=gvimdiff2` will open `gvim` with three columns (LOCAL,
+MERGED and REMOTE).