ref-filter: use contains_result enum consistently

ref-filter: use contains_result enum consistently

Commit cbc60b672 (git tag --contains: avoid stack overflow,
2014-04-24) adapted the -1/0/1 contains status into a
tri-state enum. However, some of the code still used the
numeric values, or assumed that no/yes correspond to C's
boolean true/false.

Let's switch to using the symbolic values everywhere, which
will make it easier to change them.

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