Fix dulwich._compat docstring to make pydoctor happy.
Update hooks.py

python2.4 compat for hooks.py

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Added_symbolic_ref function into porcelain module.

This implements `git symbolic-ref` command via Dulwich porcelain.
Python 2.4+ compat
Fixed porcelain test.