checkout: add --working-tree-only optionjc/checkout-working-tree-only

"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 <gitster@pobox.com>

3 files changed, 43 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 81f07c3ef2..5d583d6c96 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' requires a 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);
@@ -1164,6 +1170,7 @@ 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..9ad74cebb2 100755
--- a/t/t2022-checkout-paths.sh
+++ b/t/t2022-checkout-paths.sh
@@ -78,4 +78,24 @@ 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 line >>file1 &&
+	echo exists >file3 &&
+	git add file3 &&
+	git commit -a -m "another commit" &&
+	git checkout next &&
+
+	! grep "another line" file1 &&
+	git checkout --working-tree-only pu file1 file3 &&
+	grep "another line" file1 &&
+	test_must_fail git grep --cached "another line" file1 &&
+
+	grep exists file3 &&
+	git ls-files file3 >actual &&
+	>expect &&
+	test_cmp expect actual
+'
+
 test_done