check-docs: list git-gui as a command

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>