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: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@297849 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,hsharsha/depot_tools,duongbaoduy/gtools,aleonliao/depot_tools,duongbaoduy/gtools,aleonliao/depot_tools,hsharsha/depot_tools,duanwujie/depot_tools,aleonliao/depot_tools,duanwujie/depot_tools,CoherentLabs/depot_tools,CoherentLabs/depot_tools,duongbaoduy/gtools,primiano/depot_tools,primiano/depot_tools,hsharsha/depot_tools,duanwujie/depot_tools