repo sync: Don't complain about deleted remote branches

[abhinav]

Right now, if you run git pull or git fetch with --prune before calling repo sync, it will complain that the remote tracking branch is missing. It's only an error, not a warning, but it can still be confusing.

This makes repo sync check if the remote tracking branch exists before trying to delete it, making it a no-op if the branch has already been deleted.

[abhinav]

This change is part of the following stack:

• 395 ◀

_{Change managed by git-spice.}