The old `allocfmt` is of no use to callers, as they are not able to free
the returned buffer. Export a new API that returns a static string that
doesn't need to be freed.
As suggested by carlosmn, git_oid_ncmp would probably
be a better name than git_oid_match, for it does the same
as git_oid_cmp but only up to a certain amount of hex digits.
