rMOZILLACENTRAL65bd52f60c9f

Bug 1354625 - Use mozilla-unified for upstream for all Firefox builds. r=aki, a=NPOTB

Without the use of an upstream the first repo cloned on a machine will
be cached. If a subsequent job references a different repo, it may have
to pull thousands of changesets because those changesets aren't part of
the initially-cloned repo. This is why the --upstream feature to
robustclone exists and is why it uses the mozilla-unified repo.

The mozilla-unified repo is a superset of central, aurora, beta, release,
etc.