This makes uploading to Gerrit with `git cl` more similar to uploading to Rietveld, by uploading a squashed commit containing the diff to the newest common ancestor.

Uploaded commits are stored in refs/heads/git_cl_uploads/<branch> (and their commit message is used for the squash commit), which allows amending the commit message to change it on the uploaded CL, and looking through the reflog to find old uploaded versions.

BUG=none

Review URL: https://codereview.chromium.org/200023004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294077 0039d316-1c4b-4281-b951-d872f2087c98