Drop redundant methods. The ncmp method is now public

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.

Changed return value of git_oid_match to be consistent with the other compare methods (0 means oids match). Added method to compare prefixes of hex formatted oids.

Added error for ambiguous oid prefixes. Added methods to compare the first nth hexadecimal characters (i.e. packets of 4 bits) of OIDs.