Separate tags from branches

This is needed so we can check out branches (REL1_XX) which exist on
the extensions.  Per-version tags don't exist on the extensions.

Note that this breaks the tests since we have separate slots for
branches and tags.

Change-Id: I250d2573909c39b2ed2c1e23ae5d18cdf4f45c93
Merge "Separate tags from branches"