Update prebuilt.py to upload correctly when tracking branch is a revision.

If the tracking branch is a revision, we shouldn't push there. We need to
look in the manifest for the default branch and use that instead.

TEST=Run prebuilt.py when tracking branch is a revision and verify it pushes
     to the right branch (i.e. the default branch defined in the manifest).

Change-Id: I3cdab81a9d8613bc30825b221ee169fb03211417
Reviewed-on: http://gerrit.chromium.org/gerrit/1749
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David James <davidjames@chromium.org>
1 file changed