use skip_prefix to avoid repeated calculations

use skip_prefix to avoid repeated calculations

In some cases, we use starts_with to check for a prefix, and
then use an already-calculated prefix length to advance a
pointer past the prefix. There are no magic numbers or
duplicated strings here, but we can still make the code
simpler and more obvious by using skip_prefix.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>