[PATCH v5 1/2] reset: enable '-' short-hand for previous branch

[Sudhanshu Shekhar <sudshekhar02@gmail.com>]

git reset '-' will reset to the previous branch. To reset a file named
"-" use either "git reset ./-" or "git reset -- -".

Change error message to treat single "-" as an ambigous revision or
path rather than a bad flag.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Sudhanshu Shekhar <sudshekhar02@gmail.com>
---
I have changed the logic to ensure argv[0] isn't changed at any point.
Creating a modified_argv0 variable let's me do that.

I couldn't think of any other way to achieve this, apart from changing things
directly in the sha1_name.c file (like Junio's changes). Please let me know
if some further changes are needed.

 builtin/reset.c | 17 +++++++++++++----
 setup.c         |  2 +-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/builtin/reset.c b/builtin/reset.c
index 4c08ddc..bc50e14 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -192,6 +192,7 @@ static void parse_args(struct pathspec *pathspec,
 {
 	const char *rev = "HEAD";
 	unsigned char unused[20];
+	const char *modified_argv0 = argv[0];
 	/*
 	 * Possible arguments are:
 	 *
@@ -205,10 +206,17 @@ static void parse_args(struct pathspec *pathspec,
 	 */
 
 	if (argv[0]) {
+		if (!strcmp(argv[0], "-")) {
+			modified_argv0 = "@{-1}";
+		}
+		else {
+			modified_argv0 = argv[0];
+		}
+
 		if (!strcmp(argv[0], "--")) {
 			argv++; /* reset to HEAD, possibly with paths */
 		} else if (argv[1] && !strcmp(argv[1], "--")) {
-			rev = argv[0];
+			rev = modified_argv0;
 			argv += 2;
 		}
 		/*
@@ -216,14 +224,15 @@ static void parse_args(struct pathspec *pathspec,
 		 * has to be unambiguous. If there is a single argument, it
 		 * can not be a tree
 		 */
-		else if ((!argv[1] && !get_sha1_committish(argv[0], unused)) ||
-			 (argv[1] && !get_sha1_treeish(argv[0], unused))) {
+		else if ((!argv[1] && !get_sha1_committish(modified_argv0, unused)) ||
+			 (argv[1] && !get_sha1_treeish(modified_argv0, unused))) {
 			/*
 			 * Ok, argv[0] looks like a commit/tree; it should not
 			 * be a filename.
 			 */
 			verify_non_filename(prefix, argv[0]);
-			rev = *argv++;
+			rev = modified_argv0;
+			argv++;
 		} else {
 			/* Otherwise we treat this as a filename */
 			verify_filename(prefix, argv[0], 1);
diff --git a/setup.c b/setup.c
index 979b13f..b621b62 100644
--- a/setup.c
+++ b/setup.c
@@ -200,7 +200,7 @@ void verify_filename(const char *prefix,
 		     int diagnose_misspelt_rev)
 {
 	if (*arg == '-')
-		die("bad flag '%s' used after filename", arg);
+		die("ambiguous argument '%s': unknown revision or path", arg);
 	if (check_filename(prefix, arg))
 		return;
 	die_verify_filename(prefix, arg, diagnose_misspelt_rev);
-- 
2.3.1.277.gd67f9d5.dirty