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>

1 files changed, 8 insertions, 1 deletions

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(),
 	};