title: ALL commits are duplicated after BFG (history editing)
body: <p>I didn't know about the clean way to remove big/sensitive files with BFG, and missed a critical step:
  <code>git clone --mirror git://example.com/some-big-repo.git</code></p>
  
  <p>Which, when I tried to push to remote, lead to history conflicts, which I naively resolved with 
  <code>git pull origin master  --allow-unrelated-histories</code>, merged a few conflicts, then pushed.</p>
  
  <p>this duplicated the commits, sometimes 5-10 times.</p>
  
  <p>Since I'm alone working on this repo, I neither have an option to clean this to the ground and restart from a more reasonable copy, nor I have to worry that different commits are different, I'm sure they are identical.</p>
  
  <p>Is there a brute-force command to erase all commits that are identical in all aspects but the hashes?</p>