git_oid_ncmp was making some assumptions about the length of
the data - this shifts the check to the top of the loop so it
will work more robustly, limits the max, and adds some tests
to verify the functionality.
This includes a few cleanups that came up while converting
these files.
This commit introduces a could new git error classes, including
the catchall class: GITERR_INVALID which I'm using as the class
for invalid and out of range values which are detected at too low
a level of library to use a higher level classification. For
example, an overflow error in parsing an integer or a bad letter
in parsing an OID string would generate an error in this class.
