cat-file: add mailmap support to --batch-check option

Even though the cat-file command with `--batch-check` option does not
complain when `--use-mailmap` option is given, the latter option is
ignored. Compute the size of the object after replacing the idents and
report it instead.

In order to make `--batch-check` option honour the mailmap mechanism we
have to read the contents of the commit/tag object.

There were two ways to do it:

1. Make two calls to `oid_object_info_extended()`. If `--use-mailmap`
   option is given, the first call will get us the type of the object
   and second call will only be made if the object type is either a
   commit or tag to get the contents of the object.

2. Make one call to `oid_object_info_extended()` to get the type of the
   object. Then, if the object type is either of commit or tag, make a
   call to `repo_read_object_file()` to read the contents of the object.

I benchmarked the following command with both the above approaches and
compared against the current implementation where `--use-mailmap`
option is ignored:

`git cat-file --use-mailmap --batch-all-objects --batch-check --buffer
--unordered`

The results can be summarized as follows:
                       Time (mean ± σ)
default               827.7 ms ± 104.8 ms
first approach        6.197 s ± 0.093 s
second approach       1.975 s ± 0.217 s

Since, the second approach is faster than the first one, I implemented
it in this patch.

The command git cat-file can now use the mailmap mechanism to replace
idents with canonical versions for commit and tag objects. There are
several options like `--batch`, `--batch-check` and `--batch-command`
that can be combined with `--use-mailmap`. But the documentation for
`--batch`, `--batch-check` and `--batch-command` doesn't say so. This
patch fixes that documentation.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 36 insertions, 13 deletions

diff --git a/Documentation/git-cat-file.txt b/Documentation/git-cat-file.txt
index f82d702d6b..830f0a2eff 100644
--- a/Documentation/git-cat-file.txt
+++ b/Documentation/git-cat-file.txt
@@ -91,26 +91,49 @@ OPTIONS
 --batch::
 --batch=<format>::
 	Print object information and contents for each object provided
-	on stdin.  May not be combined with any other options or arguments
-	except `--textconv` or `--filters`, in which case the input lines
-	also need to specify the path, separated by whitespace.  See the
-	section `BATCH OUTPUT` below for details.
+	on stdin. May not be combined with any other options or arguments
+	except `--textconv`, `--filters`, or `--use-mailmap`.
+	+
+	* When used with `--textconv` or `--filters`, the input lines
+	  must specify the path, separated by whitespace. See the section
+	  `BATCH OUTPUT` below for details.
+	+
+	* When used with `--use-mailmap`, for commit and tag objects, the
+	  contents part of the output shows the identities replaced using the
+	  mailmap mechanism, while the information part of the output shows
+	  the size of the object as if it actually recorded the replacement
+	  identities.
 
 --batch-check::
 --batch-check=<format>::
-	Print object information for each object provided on stdin.  May
-	not be combined with any other options or arguments except
-	`--textconv` or `--filters`, in which case the input lines also
-	need to specify the path, separated by whitespace.  See the
-	section `BATCH OUTPUT` below for details.
+	Print object information for each object provided on stdin. May not be
+	combined with any other options or arguments except `--textconv`, `--filters`
+	or `--use-mailmap`.
+	+
+	* When used with `--textconv` or `--filters`, the input lines must
+	 specify the path, separated by whitespace. See the section
+	 `BATCH OUTPUT` below for details.
+	+
+	* When used with `--use-mailmap`, for commit and tag objects, the
+	  printed object information shows the size of the object as if the
+	  identities recorded in it were replaced by the mailmap mechanism.
 
 --batch-command::
 --batch-command=<format>::
 	Enter a command mode that reads commands and arguments from stdin. May
-	only be combined with `--buffer`, `--textconv` or `--filters`. In the
-	case of `--textconv` or `--filters`, the input lines also need to specify
-	the path, separated by whitespace. See the section `BATCH OUTPUT` below
-	for details.
+	only be combined with `--buffer`, `--textconv`, `--use-mailmap` or
+	`--filters`.
+	+
+	* When used with `--textconv` or `--filters`, the input lines must
+	  specify the path, separated by whitespace. See the section
+	  `BATCH OUTPUT` below for details.
+	+
+	* When used with `--use-mailmap`, for commit and tag objects, the
+	  `contents` command shows the identities replaced using the
+	  mailmap mechanism, while the `info` command shows the size
+	  of the object as if it actually recorded the replacement
+	  identities.
+
 +
 `--batch-command` recognizes the following commands:
 +