Provide a flexible and performant backup and restore method for project data on GitLab
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### Proposal
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A customer on ~"GitLab Dedicated" are looking for a way to backup their repositories including MR metadata.
This is currently not possible using `git` commands and [project export and import by files](https://docs.gitlab.com/user/project/settings/import_export/#export-a-project-and-its-data) is too slow to be useful and is also not considered a backup tool.
Ideally the backup should include the git repository, branches, commit history and MR metadata, so that it could be restored to another GitLab project with all this information included.
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