gitaly: Add functionality to update default branch
Transactions and log entries in the write-ahead log now have fields for considering updates to the default branch. Let's add code to consider these fields and update the default branch.
We borrow the main crux of this from
localrepo.Repo::setDefaultBranchWithTransaction
and modify as
necessary. We also add helper functions to the test code to check for
the default branch.
Closes #4738 (closed)