repo.descendant_of() always returns True on invalid SHA's

[cjerdonek]

I'm getting that if you pass a 40-character string SHA to repo.descendant_of() that doesn't correspond to a commit (for either argument), then the function always returns True. I haven't verified, but it looks like it could be because of this line:

pygit2/src/repository.c

Line 593 in 976450a

return PyBool_FromLong(git_graph_descendant_of(self->repo, &oid1, &oid2));

The PyBool_FromLong() converts an error code to True. Instead, an error code should result in an exception IMO.

[jdavid]

Good catch!

PR maybe?