git: avoid magic number with skip_prefix

After handling options, any leftover arguments should be
commands. However, we pass through "--help" and "--version",
so that we convert them into "git help" and "git version"
respectively.

This is a straightforward use of skip_prefix to avoid a
magic number, but while we are there, it is worth adding a
comment to explain this otherwise confusing behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -588,8 +588,8 @@ int main(int argc, char **av)
 	argc--;
 	handle_options(&argv, &argc, NULL);
 	if (argc > 0) {
-		if (starts_with(argv[0], "--"))
-			argv[0] += 2;
+		/* translate --help and --version into commands */
+		skip_prefix(argv[0], "--", &argv[0]);
 	} else {
 		/* The user didn't specify a command; give them help */
 		commit_pager_choice();