convert trivial uses of strncmp() to skip_prefix()

convert trivial uses of strncmp() to skip_prefix()

As with the previous patch, using skip_prefix() is more readable and
less error-prone than a raw strncmp(), because it avoids a
manually-computed length. These cases differ from the previous patch
that uses starts_with() because they care about the value after the
matched prefix.

We can convert these to use skip_prefix() by introducing an extra
variable to hold the out-pointer.

Note in the case in ws.c that to get rid of the magic number "9"
completely, we also switch out "len" for recomputing the pointer
difference. These are equivalent because "len" is always "ep - string".

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