parse: prefer bool to int for boolean returns

parse: prefer bool to int for boolean returns

All of the integer parsing functions in parse.[ch] return an int that is
"0" for failure or "1" for success. Since most of the other functions in
Git use "0" for success and "-1" for failure, this can be confusing.
Let's switch the return types to bool to make it clear that we are using
this other convention. Callers should not need to update at all.

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