This changes the 'empty' branch detection to compare the commit trees instead
of the commit hashes. This avoids empty-branch detection when you have multiple
renames on the branch that end up canceling eachother out. Previously you'd
end up with the rename-commits, in order, in your rebased branch, even though
when you run `git upstream-diff` you get nothing.

R=agable@chromium.org, jochen@chromium.org, vadimsh@chromium.org
BUG=438208

Review URL: https://codereview.chromium.org/1497313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297849 0039d316-1c4b-4281-b951-d872f2087c98