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>

remote-curl.c

@@ -1255,8 +1255,9 @@ static void parse_push(struct strbuf *buf)
 	int ret;
 
 	do {
-		if (starts_with(buf->buf, "push "))
-			argv_array_push(&specs, buf->buf + 5);
+		const char *arg;
+		if (skip_prefix(buf->buf, "push ", &arg))
+			argv_array_push(&specs, arg);
 		else
 			die(_("http transport does not support %s"), buf->buf);