Support reftable in default branch modification

The localrepo.SetDefaultBranch() function sets the default branch of a repository by creating a HEAD.lock file and manually editing the HEAD file. This is because we want to incorporate modifying the HEAD file via a transaction (so nodes can vote for the change).

The tests which should be fixed after fixing this issue:

• TestCreateRepository_successful
• TestManager_RemoveRepository
• TestManager_Restore_latest
• TestManager_Restore_specific
• TestRepo_FetchBundle/HEAD_update
• TestRepo_HeadReference
• TestRepo_SetDefaultBranch
• TestServer_FetchBundle_success
• TestWriteRef/update_default_branch
• TestRepo_ContainsRef/HEAD_is_locked_by_SetDefaultBranch