Enable branch creation as part of syncing

### Is your feature request related to a problem?

In cases where the fork branch that corresponds to a mirror has been renamed or deleted, the syncing function will fail to execute instead of creating the desired branch.

### Describe the solution you'd like

During syncing, any missing fork branch should be recreated to allow the sync to continue and provide a way for users to reset a conflicted fork branch. This change would also make it unnecessary to create the fork branch during mirror creation so that logic should be removed.

### Describe alternatives you've considered

Alternatively the deletion or renaming of a fork branch could be disallowed through protections on the fork or could result in the corresponding mirror being renamed / deleted.

### Additional context

There is currently a bug detailed in #445 that makes this situation more common.

Start by tracing the syncing flow and the mirror-creation flow, then review issue #445 for the related branch condition. Done means syncing recreates a missing fork branch, can continue afterward, and no longer creates that branch during mirror creation; verify both paths with the repository's existing checks.

Written by the indexing model from the issue text.