meaning of HEAD in context of filter-branch

I tried to process some files that changed between each commit using
the command

git filter-branch --tree-filter 'git diff --name-only HEAD^.. | foo

however it seems that HEAD is fixed to mean the HEAD of the current
tip before starting git-filter-branch, by trying like this

$ git rev-list -1 HEAD^
2bf95faed2d51c338ea6fa93e7bb6cc9c1bc759b

git filter-branch --tree-filter 'git rev-list -1 HEAD^ >>/tmp/sha' -- --all
$ cat /tmp/sha
2bf95faed2d51c338ea6fa93e7bb6cc9c1bc759b
2bf95faed2d51c338ea6fa93e7bb6cc9c1bc759b
2bf95faed2d51c338ea6fa93e7bb6cc9c1bc759b
2bf95faed2d51c338ea6fa93e7bb6cc9c1bc759b


how to get the HEAD during filter-branch or other workaround?

-- 
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