C: use skip_prefix() to avoid hardcoded string length

We often skip an optional prefix in a string with a hardcoded
constant, e.g.

	if (starts_with(string, "prefix"))
		string += 6;

which is less error prone when written

	skip_prefix(string, "prefix", &string);

Note that this changes a few error messages from "git reflog expire
--expire=nonsense.timestamp", which used to complain by saying

    '--expire=nonsense.timestamp' is not a valid timestamp

but with this change, we say

    'nonsense.timestamp' is not a valid timestamp

which is more technically correct (the string with --expire= as
a prefix obviously cannot be a valid timestamp, but the error is
about the part of the input without that prefix).

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

builtin/fast-export.c

@@ -870,8 +870,7 @@ static void handle_tag(const char *name, struct tag *tag)
 		printf("reset %s\nfrom %s\n\n",
 		       name, oid_to_hex(&null_oid));
 	}
-	if (starts_with(name, "refs/tags/"))
-		name += 10;
+	skip_prefix(name, "refs/tags/", &name);
 	printf("tag %s\n", name);
 	if (mark_tags) {
 		mark_next_object(&tag->object);