Support shallow clones from dulwich server

https://www.kernel.org/pub/software/scm/git/docs/v1.7.0.5/technical/shallow.txt

https://github.com/git/git/blob/master/Documentation/technical/pack-protocol.txt

https://www.kernel.org/pub/software/scm/git/docs/git-clone.html

https://github.com/git/git/blob/master/shallow.c

etc