The New Review Request page wasn't properly filtering commits by branch

for Subversion repositories. It was assuming it could start at a

revision and pass that to the equivalent of svn log. However, this

ended up picking up commits that existed on other branches as well.
The Repository Commits API for commits now accepts a branch= parameter,

which is the branch ID returned by the Repository Branches API. This is

passed through to get_commits(), which will then filter by that branch.
Through this, it's possible to properly limit the commits shown to those

on the branch. We'll also be able to make use of this later for other

repositories, if they suffer the same problem.

Set up a repository with different commits on different branches, and

some shared history.
Before this change, I was getting the wrong commits on different selected

branches.
After, I was seeing only the commits I'd expect, which matched what 'svn log'

was showing for those branches.