Consistently use "superproject" instead of "supermodule"

Consistently use "superproject" instead of "supermodule"

We fairly consistently say "superproject" and never "supermodule" these
days. But there are seven occurrences of "supermodule" left in the current
work tree. Three appear in Release Notes for 1.5.3 and 1.7.7, three in
test names and one in a C-code comment.

Replace all occurrences of "supermodule" outside of the Release Notes
(which shouldn't be changed after the fact) with "superproject" for
consistency.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>