use strip_suffix instead of ends_with in simple cases

use strip_suffix instead of ends_with in simple cases

When stripping a suffix like:

  if (ends_with(str, "foo"))
	buf = xmemdupz(str, strlen(str) - 3);

we can instead use strip_suffix to avoid the constant 3,
which must match the literal "foo" (we sometimes use
strlen("foo") instead, but that means we are repeating
ourselves). The example above becomes:

  if (strip_suffix(str, "foo", &len))
	buf = xmemdupz(str, len);

This also saves a strlen(), since we calculate the string
length when detecting the suffix.

Note that in some cases we also switch from xstrndup to
xmemdupz, which saves a further strlen call.

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