check-docs: factor out command-list
check-docs: factor out command-list

The check-docs command list is composed from several
Makefile variables plus some special cases. Let's make the
meaning of the list more obvious and avoid repeating
ourselves by factoring it out.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git