Rebase may cause commits to disappear if they have no commits pointing to it

[taneliang]

When rebasing commits in a branch, commits may disappear if they are no longer a parent of a commit with >=1 branch pointing to it.

Steps to repro

1. Given this commit graph
   
2. Use Stack Attack to rebase b86cede onto faeff9f.
3. 4628f57 should disappear, since there are no more branches pointing to it or its children.

Steps

• Confirm that this issue is actually a thing (I didn't test it LOL)
• Generate Stack Attack branch for new branch tips that don't have commits pointing to them