-- >8 --
Subject: [PATCH] checkout: add --working-tree-only option

"git checkout <tree-ish> <pathspec>" has always copied the blob from
the tree-ish to the index before checking them out to the working tree.

Some users may want to grab a blob out of a tree-ish directly to the
working tree, without updating the index, so that "git diff" can be
used to assess the damage and adjust the file contents taken from a
different branch to be more appropriate for the current branch.

The new option "--working-tree-only" allows exactly that.

In the hindsight, when a command works on the working tree and/or
the index, the usual convention is:

- with no other option, the command works only on the working tree;

- with "--cached" option, the command works only on the index; and

- with "--index" option, the command works on both the working tree
  and the index.

So we probably should have triggered the default behaviour under the
"--index" option, and triggered this "--working-tree-only" mode of
behaviour when "--index" option is not given.  From the same point
of view, "git checkout --cached <tree-ish> <pathspec>" would have
done the same as "git reset <tree-ish> <pathspec>" would do.  And
that may have made the command set a bit more consistent.

But that is merely a hindsight being 20/20, oh well.

Signed-off-by: Junio C Hamano <redacted>
---
Documentation/git-checkout.txt | 22 +++++++++++++++-------
builtin/checkout.c             | 10 +++++++++-
t/t2022-checkout-paths.sh      | 21 +++++++++++++++++++++
3 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/Documentation/git-checkout.txt 

b/Documentation/git-checkout.txt
index 8e2c0662dd..201677752e 100644

--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt

@@ -14,6 +14,7 @@ SYNOPSIS

'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] 
[<start_point>]
'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] 
[--] <paths>...
'git checkout' [-p|--patch] [<tree-ish>] [--] [<paths>...]
+'git checkout' --working-tree-only <tree-ish> [--] [<paths>...]

DESCRIPTION
-----------

@@ -81,13 +82,14 @@ Omitting <branch> detaches HEAD at the tip of the 

current branch.
'git checkout' [-p|--patch] [<tree-ish>] [--] <pathspec>...::

 When <paths> or `--patch` are given, 'git checkout' does *not*
- switch branches.  It updates the named paths in the working tree
- from the index file or from a named <tree-ish> (most often a
- commit).  In this case, the `-b` and `--track` options are
- meaningless and giving either of them results in an error.  The
- <tree-ish> argument can be used to specify a specific tree-ish
- (i.e.  commit, tag or tree) to update the index for the given
- paths before updating the working tree.
+ switch branches.  In this case, the `-b` and `--track` options
+ are meaningless and giving either of them results in an error.
++
+The command checks out blobs for paths that match the given
+<pathspec> from the index to the working tree.  When an optional
+<tree-ish> is given, the blobs for paths that match the given
+<pathspec> are copied from the <tree-ish> to the index before
+they are checked out of the index.
+
'git checkout' with <paths> or `--patch` is used to restore modified or
deleted paths to their original contents from the index or replace paths

@@ -101,6 +103,12 @@ specific side of the merge can be checked out of the 

index by
using `--ours` or `--theirs`.  With `-m`, changes made to the working tree
file can be discarded to re-create the original conflicted merge result.

+'git checkout' --working-tree-only <tree-ish> [--] <pathspec>...::
+ Similar to `git checkout <tree-ish> [--] <pathspec>`, but
+ the index file is left in the same state as it was before
+ running this command.

+
+
OPTIONS
-------
-q::

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 9b2a5b31d4..d214e99521 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c

@@ -37,6 +37,7 @@ struct checkout_opts {
 int overwrite_ignore;
 int ignore_skipworktree;
 int ignore_other_worktrees;
+ int no_index;
 int show_progress;

 const char *new_branch;

@@ -268,6 +269,9 @@ static int checkout_paths(const struct checkout_opts 

*opts,
 die(_("Cannot update paths and switch to branch '%s' at the same time."),
     opts->new_branch);

+ if (opts->no_index && !opts->source_tree)
+ die(_("'--working-tree-only' cannot be used without tree-ish"));
+
 if (opts->patch_mode)
 return run_add_interactive(revision, "--patch=checkout",
    &opts->pathspec);

@@ -370,7 +374,9 @@ static int checkout_paths(const struct checkout_opts 

*opts,
 }
 }

- if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+ if (opts->no_index)
+ ; /* discard the in-core index */
+ else if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
 die(_("unable to write new index file"));

 read_ref_full("HEAD", 0, rev.hash, NULL);

@@ -1161,6 +1167,8 @@ int cmd_checkout(int argc, const char **argv, const 

char *prefix)
 OPT_BOOL(0, "ignore-other-worktrees", &opts.ignore_other_worktrees,
 N_("do not check if another worktree is holding the given ref")),
 OPT_BOOL(0, "progress", &opts.show_progress, N_("force progress 
reporting")),
+ OPT_BOOL(0, "working-tree-only", &opts.no_index, N_("checkout to working 
tree only without touching the index")),
+
 OPT_END(),
 };

diff --git a/t/t2022-checkout-paths.sh b/t/t2022-checkout-paths.sh
index f46d0499bc..8ea2e34c90 100755
--- a/t/t2022-checkout-paths.sh
+++ b/t/t2022-checkout-paths.sh

@@ -78,4 +78,25 @@ test_expect_success 'do not touch files that are 

already up-to-date' '
 test_cmp expect actual
'

+test_expect_success 'working-tree-only option leaves checked out files 
unadded' '
+ git reset --hard &&
+ git checkout -b pu next &&
+ echo another >>file1 &&
+ echo exists >file3 &&
+ git add file3 &&
+ git commit -a -m another &&
+ git checkout next &&
+
+ ! grep another file1 &&
+ git checkout --working-tree-only pu file1 file3 &&
+ grep another file1 &&
+ test_must_fail git grep --cached another file1 &&
+
+ grep exists file3 &&
+ git ls-files file3 >actual &&
+ >expect &&
+ test_cmp expect actual
+
+'
+
test_done
-- 
2.13.0-rc0-309-gb63395ed9e