client fetch errors when remote repo is empty

[milki]

If a remote git repo is empty (newly created with git init --bare), when dulwich.client tries to fetch from the repo, _read_refs will error.

Examples:

https://gist.github.com/3823386

This happens because _read_refs does not enter the for loop where it will set server_capabilities to something not None and set(None) errors.

_read_refs should return something sane in this case and dependents should handle this case.

[milki (milki-p)]

Change read_refs to return None, set([]) and check for this in send_pack

Note: This doesn't patch the http client

[Jelmer Vernooij (jelmer)]

Merged, thanks!