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, 12 insertions, 0 deletions

diff --git a/Documentation/gitcli.txt b/Documentation/gitcli.txt
index 592e06d..225513e 100644
--- a/Documentation/gitcli.txt
+++ b/Documentation/gitcli.txt
@@ -209,6 +209,18 @@ See also http://marc.info/?l=git&m=116563135620359 and
 http://marc.info/?l=git&m=119150393620273 for further
 information.
 
+Some other commands that also work on files in the working tree and/or
+in the index can take `--staged` and/or `--worktree`.
+
+* `--staged` is exactly like `--cached`, which is used to ask a
+  command to only work on the index, not the working tree.
+
+* `--worktree` is the opposite, to ask a command to work on the
+  working tree only, not the index.
+
+* The two options can be specified together to ask a command to work
+  on both the index and the working tree.
+
 GIT
 ---
 Part of the linkgit:git[1] suite