Add retry for all git operations that speak to a remote.  This
should smooth out issues with the git/gerrit-on-borg service.

Review URL: https://codereview.chromium.org/26234004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@229219 0039d316-1c4b-4281-b951-d872f2087c98