RFH: is the current file available to a custom merge driver?

In writing a custom merge driver to support $dayjob
requirements I've failed to find a simple way of getting
the current file name that the driver is supposed to
be merging.

I of course have the "ancestor", "ours", and "theirs"
temporary files, but for Ui purposes I'd like to know
the filename as well.

As a workaround, I'm grepping for the git-hash-object
of %A (aka "ours) from the output of "git ls-tree -r HEAD".

I've examined the environment, and saw nothing relevant.
"git-ls-files -u" isn't useful, since the items aren't
unmerged yet.

Can anyone offer hints/help/tips?

Thanks - Eric

git version 1.7.0.2.msysgit.0