Remove an unreferenced commit from git repository

Question

I have a git commit history like this:

          U
         / 
A---B---C---D---E master

Nothing points to the commit U, but I

Answer 1

Eventually it will be cleaned up by git, but you can look into git gc to actually force a garbage collect.

The clean command is different and won't take care of this for you.

Answer 2

As described here, simply use

git reflog expire --expire-unreachable=now --all
git gc --prune=now

git reflog expire --expire-unreachable=now --all removes all references of unreachable commits in reflog.

git gc --prune=now removes the commits themselves.

Attention: Only using git gc --prune=now will not work since those commits are still referenced in the reflog. Therefore, clearing the reflog is mandatory.