git-commit broken with GIT_INDEX_FILE

hey All.

i'm maintaining my own git porcelainen, and as part of that had to
(re-)implement my own commit command:
   http://blogs.gnome.org/timj/2007/10/13/13102007-yummy-yummy-porcelain-version-08/

i've read through git-commit and cg-commit, and would really like to avoid
reimplementing the hook and utf8 encoding handling. thus, my tool calls
git-commit once it has created an index file with all the files that
need comitting.
ideally, it'd construct a temporary index, e.g. $GIT_DIR/next-index$$ and
then invoke:
   GIT_INDEX_FILE=$GIT_DIR/next-index$$ git-commit -F commitmsg.txt
this is not working as expected however.

when GIT_INDEX_FILE=tmpindex is specified for git-commit-1.5.3.4, it will
construct a commit message for tmpindex, but it'll still use the standard
index file $GIT_DIR/index for git-write-tree.

(so my current implementation saves and restores the standard index file
around committs.)

for consistent commit messages, the index file used for commit message
construction should be the same index used for git-write-tree.
ideally however, GIT_INDEX_FILE=tmpindex git-commit would work correctly
in using tmpindex for git-write-tree as well.

---
ciaoTJ