avoid using skip_prefix as a boolean

avoid using skip_prefix as a boolean

There's no point in using:

  if (skip_prefix(buf, "foo"))

over

  if (starts_with(buf, "foo"))

as the point of skip_prefix is to return a pointer to the
data after the prefix. Using starts_with is more readable,
and will make refactoring skip_prefix easier.

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