git.git uses an inlined hashcmp function instead of memcmp, since it
performes much better when comparing hashes (most hashes compared
diverge within the first byte).

Measurements and rationale for the curious reader:
http://thread.gmane.org/gmane.comp.version-control.git/172286

odb: add some documentation to the foreach() test

Add a struct for network callbacks

Yes, we can get the repository from the owner of the object, but having
it marked explicitly makes the API more consistent.

Add support for push-urls

Fix/issue 787