Add mirror details and cleanup warning

What does this MR do?

While testing the repo resize process I noticed the example clone command was missing the --mirror option referenced in the preceding description. Without this option internal refs are not cleaned up. When this option is specified an additional step is required to remove the mirror flag before force pushing, else an error occurs.

A warning has also been added highlighting that the cleanup of internal refs will remove the commits, pipelines and changes details from the MR UI as the commits refs stored in the database are no longer valid. This will hopefully help avoid surprises such as in #355465 (closed).

Related issues

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