The editor program to let the user edit the log message used to
get GIT_INDEX_FILE environment variable pointing at the right
file, but this was lost when git-commit was rewritten in C.
Signed-off-by: Pierre Habouzit <madcoder@debian.org> Acked-by: Kristian Høgsberg <krh@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>