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, 4 insertions, 3 deletions

diff --git a/Documentation/git-reset.txt b/Documentation/git-reset.txt
index cbf901efb4..c25f8a95b9 100644
--- a/Documentation/git-reset.txt
+++ b/Documentation/git-reset.txt
@@ -25,7 +25,8 @@ The `<tree-ish>`/`<commit>` defaults to `HEAD` in all forms.
 	the current branch.)
 +
 This means that `git reset <paths>` is the opposite of `git add
-<paths>`.
+<paths>`. This command is equivalent to
+`git restore [--source=<tree-ish>] --staged <paths>...`.
 +
 After running `git reset <paths>` to update the index entry, you can
 use linkgit:git-checkout[1] to check the contents out of the index to
@@ -86,8 +87,8 @@ but carries forward unmerged index entries.
 	changes, reset is aborted.
 --
 
-If you want to undo a commit other than the latest on a branch,
-linkgit:git-revert[1] is your friend.
+See "Reset, restore and revert" in linkgit:git[1] for the differences
+between the three commands.
 
 
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