remote: force completing --mirror= instead of --mirror

"git remote --mirror" is a special case. Technically it is possible to
specify --mirror without any argument. But we will get a "dangerous,
deprecated!" warning in that case.

This new parse-opt flag allows --git-completion-helper to always
complete --mirror=, ignoring the dangerous use case.

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

parse-options.c

@@ -459,6 +459,8 @@ static int show_gitcomp(struct parse_opt_ctx_t *ctx,
 		default:
 			break;
 		}
+		if (opts->flags & PARSE_OPT_COMP_ARG)
+			suffix = "=";
 		printf(" --%s%s", opts->long_name, suffix);
 	}
 	fputc('\n', stdout);