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).

Reviewed-on: http://gerrit.chromium.org/gerrit/1702
Reviewed-by: Nick Sanders <nsanders@chromium.org>
Tested-by: David James <davidjames@chromium.org>
(cherry picked from commit d611f3d89a40ca74bb9218f341259a9a1b6f351b)

Change-Id: I9d934fb175bf59b20d6c244f0526edd9b4e20004
Reviewed-on: http://gerrit.chromium.org/gerrit/1746
Reviewed-by: <scottz@chromium.org>
Tested-by: David James <davidjames@chromium.org>
2 files changed