revision.c: leave a note for "a lone :" enhancement

revision.c: leave a note for "a lone :" enhancement

If we later add a command in the log family that by default limit
its operation to the current subdirectory, we would need to resurrect
the "a lone ':' on the command line means no pathspec whatsoever".

Now the codepath was cleaned up, we can do so in one place.  Leave a
note to mark where it is for later generations.

Signed-off-by: Junio C Hamano <gitster@pobox.com>