fast-import [mis?]-honors core.ignorecase

I was using a "fast-export | <filter program> | fast-import" as a much faster filter-branch --env-filter (I needed to rewrite author/committer email-addresses) and was surprised that after the pipeline was done, the new ref's trees didn't match the old ref's trees. i.e.:

  $ cmp <(git log old-ref --pretty='%T')  <(git log --pretty='%T' new-ref)

didn't return 0. Comparing one of the differing tree-pairs indicated it was due to a case-difference in a file which had been renamed. After some experimenting I finally tracked this down to having core.ignorecase set to "true" on the repo in which I was running the pipeline. By setting it to "false" the filtering pipeline completes with new-ref (and all its ancestors) being tree-identical to old-ref.

Is there any reason for fast-import to honor ignorecase?

j.