Allow keeping empty commits when deleting blobs

Proposal

With https://gitlab.com/gitlab-org/gitlab/-/issues/450701 we introduced the possibility to remove unwanted files from git history.

By default, git filter-repo prunes empty commits.

Some users use SHAs in internal processes and would benefit from having the possibility to keep empty commits. Reference: Zendesk link(internal).

Empty commits can be kept if git filter-repo is called with the option --prune-empty never. This option can be potentially exposed through a checkbox in the UI.