When merging conflicting submodule changes from a supermodule, generate
a conflict message saying what went wrong. Also leave the tree in a state
where git status shows the conflict, and git submodule status gives the user
enough information to do the merge manally. Previously this would just fail.
Signed-off-by: Finn Arne Gangstad <finnag@pvv.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>