tag: use skip_prefix instead of magic numbers

We can make the parsing of the --sort parameter a bit more
readable by having skip_prefix keep our pointer up to date.

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

builtin/tag.c

@@ -524,18 +524,14 @@ static int parse_opt_sort(const struct option *opt, const char *arg, int unset)
 	int *sort = opt->value;
 	int flags = 0;
 
-	if (*arg == '-') {
+	if (skip_prefix(arg, "-", &arg))
 		flags |= REVERSE_SORT;
-		arg++;
-	}
-	if (starts_with(arg, "version:")) {
-		*sort = VERCMP_SORT;
-		arg += 8;
-	} else if (starts_with(arg, "v:")) {
+
+	if (skip_prefix(arg, "version:", &arg) || skip_prefix(arg, "v:", &arg))
 		*sort = VERCMP_SORT;
-		arg += 2;
-	} else
+	else
 		*sort = STRCMP_SORT;
+
 	if (strcmp(arg, "refname"))
 		die(_("unsupported sort specification %s"), arg);
 	*sort |= flags;