repo sync: Fix deleting wrong tracking branch for renamed branches

[abhinav]

If a branch is renamed after submission and then merged, the tracking branch name does not match the local branch name.

This case must be correctly handled when deleting the tracking branch: if we know the upstream branch name use $remote/$upstreamBranch instead of $remote/$branch.

Also drop the outdated comment about how we detect merged branches sinc we're indeed doing both (1) and (2) now.

[abhinav]

This change is part of the following stack:

• 401 ◀

_{Change managed by git-spice.}