user.name and/or user.email may be not configured, so specify them
explictly for commit-tree.

R=iannucci@chromium.org
BUG=354276

Review URL: https://codereview.chromium.org/222103006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@261256 0039d316-1c4b-4281-b951-d872f2087c98