Support creating mirrors that are up to date with the source repository

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

When mirrors are created their main branch is always up to date with the fork they are created from but not the source project. This makes it so that often times users are required to update the fork or their mirror manually to ensure that they have the latest changes.

### Describe the solution you'd like

When a mirror is being created, private mirrors should reference the source project instead of the fork or bring the fork up to date first.

### Describe alternatives you've considered

GitHub actions could be set up on the organization that contains the forks to ensure that all forks are kept up to date.

### Additional context

This logic will need to account for situations where the source project is private and not accessible by the private mirrors GitHub app or alternatively not accessible by the user.

No files or tests are named; start by tracing the mirror-creation entry point and how it selects the fork or source project. Done means newly created private mirrors are current with the source while handling cases where the source is private or inaccessible to the GitHub App or user.

Written by the indexing model from the issue text.