check-docs: get documented command list from Makefile

check-docs: get documented command list from Makefile

The current code tries to get a list of documented commands
by doing "ls Documentation/git*txt" and culling a bunch of
special cases from the result. Looking for "git-*.txt" would
be more accurate, but would miss a few commands like
"gitweb" and "gitk".

Fortunately, Documentation/Makefile already knows what this
list is, so we can just ask it. Annoyingly, we still have to
post-process its output a little, since make will print
extra cruft like "GIT-VERSION-FILE is up to date" to stdout.

Now that our list is accurate, we can remove all of the ugly
special-cases.

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