Fix multi-rename empty branch detection.
Fix multi-rename empty branch detection.

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@297849 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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