checkout: split part of it to new command 'restore'

Previously the switching branch business of 'git checkout' becomes a
new command 'switch'. This adds the restore command for the checking
out paths path.

Similar to git-switch, a new man page is added to describe what the
command will become. The implementation will be updated shortly to
match the man page.

A couple main differences from 'git checkout <paths>':

- 'restore' by default will only update worktree. This matters more
  when --source is specified ('checkout <tree> <paths>' updates both
  worktree and index).

- 'restore --staged' can be used to restore the index. This command
  overlaps with 'git reset <paths>'.

- both worktree and index could also be restored at the same time
  (from a tree) when both --staged and --worktree are specified. This
  overlaps with 'git checkout <tree> <paths>'

- default source for restoring worktree and index is the index and
  HEAD respectively. A different (tree) source could be specified as
  with --source (*).

- when both index and worktree are restored, --source must be
  specified since the default source for these two individual targets
  are different (**)

- --no-overlay is enabled by default, if an entry is missing in the
  source, restoring means deleting the entry

(*) I originally went with --from instead of --source. I still think
  --from is a better name. The short option -f however is already
  taken by force. And I do think short option is good to have, e.g. to
  write -s@ or -s@^ instead of --source=HEAD.

(**) If you sit down and think about it, moving worktree's source from
  the index to HEAD makes sense, but nobody is really thinking it
  through when they type the commands.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/git.txt b/Documentation/git.txt
index 00156d64aa..fbed007354 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -210,6 +210,26 @@ people via patch over e-mail.
 
 include::cmds-foreignscminterface.txt[]
 
+Reset, restore and revert
+~~~~~~~~~~~~~~~~~~~~~~~~~
+There are three commands with similar names: `git reset`,
+`git restore` and `git revert`.
+
+* linkgit:git-revert[1] is about making a new commit that reverts the
+  changes made by other commits.
+
+* linkgit:git-restore[1] is about restoring files in the working tree
+  from either the index or another commit. This command does not
+  update your branch. The command can also be used to restore files in
+  the index from another commit.
+
+* linkgit:git-reset[1] is about updating your branch, moving the tip
+  in order to add or remove commits from the branch. This operation
+  changes the commit history.
++
+`git reset` can also be used to restore the index, overlapping with
+`git restore`.
+
 
 Low-level commands (plumbing)
 -----------------------------