check-docs: list git-gui as a command

git-gui is already documented and mentioned in command-list, but adding it to the Makefile makes sure it is so. We also add its alias git-citool (which is also documented). As a result, we can drop them from the special case statement that avoids them being listed as "documented but does not exist". Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>