git-checkout: fix command line parsing.

This fixes an issue when you use:

    $ git checkout -- <path1> [<paths>...]

and that <path1> can also be understood as a reference. git-checkout
mistakenly understands this as the same as:

    $ git checkout <path1> -- [<paths>...]

because parse-options was eating the '--' and the argument parser thought
he was parsing:

    $ git checkout <path1> [<paths>...]

Where there indeed is an ambiguity

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -520,7 +520,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 
 	opts.track = git_branch_track;
 
-	argc = parse_options(argc, argv, options, checkout_usage, 0);
+	argc = parse_options(argc, argv, options, checkout_usage,
+			     PARSE_OPT_KEEP_DASHDASH);
 	if (argc) {
 		arg = argv[0];
 		if (get_sha1(arg, rev))