To add to Paul's answer, I found that to ultimately recover space, I have to push HEAD to a clean repository and that trims down the size of the .git/objects/pack directory.
i.e.
$ mkdir ...ABC.git $ cd ...ABC.git $ git init --bare
After the gc prune, also do:
$ git push ...ABC.git HEAD
Then you can do
$ git clone ...ABC.git
and the size of ABC/.git is reduced
Actually, some of the time consuming steps (e.g. git gc) aren't needed with the push to clean repository, i.e.:
$ git clone --no-hardlinks /XYZ /ABC $ git filter-branch --subdirectory-filter ABC HEAD $ git reset --hard $ git push ...ABC.git HEAD