We use git hashes to point to specific revisions, e.g., in an issue tracking system. When cleaning a repo with BFG, all/many hashes change, so that information is lost.
Can BFG, as part of cleaning a repo, produce a mapping between old and new revision hashes?
It can, and it does, by default :)
Every commit has added footer in commit message Former-commit-id: <sha1>. There is also file mapping old hashes to new ones generated during cleaning. You can pass runtime parameter -private to avoid generating this footer.