The old behavior was to only populate the 'parent_base' attribute

if the 'base' of the review range did not show up with

git branch -r --contains <base>
This fails for many common environments (such as having pushed to

your private github repository prior to posting the review).

Presumably, the assumption that was made is that if the base is

present on a remote branch, it must be already available to

Review Board. With distributed version-control systems, this is

not a safe assumption to make.
With this change, we will always look up the merge base and

compare it to the review base. If they are different, we will pass

along the parent_base so that the parent diff can be generated

correctly.

1) Set up a series of patches that depended on one another. (Patch 2 depends on changes from patch 1)

2) Pushed the patches to a remote git repository

3) Ran 'rbt post patch1:patch2'
Previous behavior:

The file was not found in the repository (HTTP 400, API Error 207)

Behavior after this patch:

Review was properly uploaded.