The buildbot drops a source stamp (repo1.git@commit1 repo2.git@commit2 ...).
If CBuildbot finds the source stamp, it uses it and revs only those packages.
If this file is either missing or set to "None", it assumes a Force Build
has been pressed and keeps the old behavior (attempts to mark all).
TEST=Ran with 3 states of revisions file.
BUG=5006
Review URL: http://codereview.chromium.org/3163030
Change-Id: I226fd3bec642224b31ce51eee34d028043964943