Add automatic parent diff support for git reviews with revision-range

Review Request #2378 — Created May 25, 2011 and submitted

When using a revision-range, you need to specify a parent diff if the first revision is not available in the upstream which reviewboard can see (e.g. via gitweb).  Hence I have made this happen automatically by 
detecting whether the upstream contains that revision and then creating a parent diff from the merge-base to the first revision.

Previously it appeared that I had to do this by creating diffs manually and uploading them to the web interface, so this improves matters somewhat for me.

I believe that this should always produce diffs which apply cleanly, but any corrections are gladly accepted  :)

Patch against most recent git sources of rbtools follows, and only adds this for the git support as that's the only repo I have handy to test it with.
Tested with examples which were failing to automatically create correct diffs on our local install of reviewboard.  e.g. local changes on a branch which has an upstream associated but the start of the revision range is not yet pushed so it may not necessarily cleanly patch.
  1. Thanks. Seems reasonable. Committed to master (049f2e6).