We are being quite lenient when parsing "ACK" packets. First, we didn't
correctly verify that we're not overrunning the provided buffer length,
which we fix here by using `git__prefixncmp` instead of
`git__prefixcmp`. Second, we do not verify that the actual contents make
any sense at all, as we simply ignore errors when parsing the ACKs OID
and any unknown status strings. This may result in a parsed packet
structure with invalid contents, which is being silently passed to the
caller. This is being fixed by performing proper input validation and
checking of return codes.