This adds the functionality to try a job spread across multiple checkouts.

TEST=fixed unit tests and tested with hybrid svn-git checkouts
BUG=none

Review URL: http://codereview.chromium.org/504085

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@35168 0039d316-1c4b-4281-b951-d872f2087c98