cros_workon: Add support for revisions other than master
cros_workon: Add support for revisions other than master

This changes adds support for specifying a revision to use for cros_workon
other than master.

BUG=chromium-os:33688
TEST=used cros_workon to workon a project, repo synced, and verified the
src was not synced to master but to the revision requested.

CQ-DEPEND:I3a0166aaf4baa3af008ba24e5e887c0ea6b6ab6e

Change-Id: I53cbf2819fc557c3ce6eca08c93a57598a64d85c
Signed-off-by: Rhyland Klein <9c6f026f68ffecc44f215af186ef513cd72b6ce4@nvidia.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/30914
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

sigma/coreos-scripts,fivethreeo/scripts,zhang0137/scripts,smilart/scripts,nicescale/scripts,marineam/coreos-scripts,andrejro/coreos--scripts,crawford/scripts,endocode/scripts,smilart/scripts,BugRoger/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,mischief/scripts,gtank/scripts,andrejro/coreos--scripts,sigma/coreos-scripts,gtank/scripts,endocode/scripts,glevand/coreos--scripts,zhang0137/scripts,BugRoger/scripts,sigma/coreos-scripts,mjg59/scripts,andrejro/coreos--scripts,nicescale/scripts,mjg59/scripts,mjg59/scripts,endocode/scripts,trnubo/scripts,trnubo/scripts,fivethreeo/scripts,mischief/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,crawford/scripts,gtank/scripts,nicescale/scripts,marineam/coreos-scripts,coreos/scripts,glevand/coreos--scripts,marineam/coreos-scripts,smilart/scripts,crawford/scripts,trnubo/scripts,mischief/scripts,vmware/coreos-scripts,zhang0137/scripts,vmware/coreos-scripts,coreos/scripts