Like earlier cases, we can use skip_prefix to avoid magic
numbers that must match the length of starts_with prefixes.
However, the numbers are a little more complicated here, as
we keep parsing past the prefix. We can solve it by keeping
a running pointer as we parse; its final value is the
location we want.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>