repack: handling of .keep files

It does not seem to be consistent with the name:
"git repack -a -d" removes .keep files anyway.

How can I pin down a pack file so that it is never repacked?
(Windows doesn't like big files, they get fragmented heavily
and are hard to read and defragment. So I try keep them
relatively small).

P.S. Experimenting with the .keep-files I had a crash in git-log,
when the pack was renamed into .keep.pack, but the
index was not. git-log complained about two objects it could not
read and than crashed. It's cygwin.