query and show commits by branch

This allows the same SHA to have different builds on different branches, each
separately viewable. This is useful for expressing a "pipeline" in terms of
branches, e.g. a commit starts on branch A and progress through B and C to
master, with the build script switching on branch name.

Previously viewing each build would arbitrarily choose which branch's commit
to show.
Merge pull request #186 from vito/branch-commits

query and show commits by branch