format-patch: fix parsing of "--" on the command line

When given a pathspec that does not match any path in the current work
tree with an explicit "--":

    git format-patch <commit> -- <path>

the command still complains that <path> does not exist in the current work
tree and the user needs to explicitly specify "--" and errors out.  This
is because it incorrectly removes "--" from the command line arguments
that is later passed to setup_revisions().

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-log.c

@@ -966,7 +966,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	 */
 	argc = parse_options(argc, argv, prefix, builtin_format_patch_options,
 			     builtin_format_patch_usage,
-			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN);
+			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN |
+			     PARSE_OPT_KEEP_DASHDASH);
 
 	if (do_signoff) {
 		const char *committer;