completion: add more parameter value completion

This adds value completion for a couple more paramters. To make it
easier to maintain these hard coded lists, add a comment at the original
list/code to remind people to update git-completion.bash too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -2119,6 +2119,10 @@ static int parse_opt_patchformat(const struct option *opt, const char *arg, int
 		*opt_value = PATCH_FORMAT_HG;
 	else if (!strcmp(arg, "mboxrd"))
 		*opt_value = PATCH_FORMAT_MBOXRD;
+	/*
+	 * Please update $__git_patchformat in git-completion.bash
+	 * when you add new options
+	 */
 	else
 		return error(_("Invalid value for --patch-format: %s"), arg);
 	return 0;