To reset your Android source tree to the branches specified in your manifests,
use repo sync --current-branch -l -j 8 --force-sync.
The -l means to not connect to the network.
To only fetch the newest references without modifying the checked-out tree, use
-n instead of -l.
We also made a change to repo_update.sh that enables you to work offline after
having fetched the relevant commits once:
Look for commits locally before fetching.