Fix a breakage in git-svn.
Fix a breakage in git-svn.

We need GitClient.head_ref to be defined for diffing, but this was only
defined in the pure Git case. We now set this before determining the mode
for GitClient.

Fixes bug #1400

datjwu/rbtools,halvorlu/rbtools,clach04/rbtools,beol/rbtools,Khan/rbtools,haosdent/rbtools,datjwu/rbtools,haosdent/rbtools,clach04/rbtools_0.4.1,halvorlu/rbtools,reviewboard/rbtools,Khan/rbtools,davidt/rbtools,davidt/rbtools,bcelary/rbtools,beol/rbtools,halvorlu/rbtools,davidt/rbtools,clach04/old_rbtools_0.3.2,datjwu/rbtools,blair/pkg-rbtools,beol/rbtools,1tush/rbtools,haosdent/rbtools,reviewboard/rbtools,reviewboard/rbtools