Fix --guess-description with Git.
Fix --guess-description with Git.

A recent change to post-review broke --guess-description with Git when not
specifically using a parent branch. We now default to grabbing all logs from
the merge base (for example, "master") and using that as the description. This
covers the same range as the diff itself.

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