revision.h

This was an undocumented debugging aid that does not seem to
have come in handy in the past decade, judging from its lack
of mentions on the mailing list.

Let's drop it in the name of simplicity. This is morally a
revert of 3131b713 (Add "--show-all" revision walker flag
for debugging, 2008-02-09), but note that I did leave in the
mapping of UNINTERESTING to "^" in get_revision_mark(). I
don't think this would be possible to trigger with the
current code, but it's the only sensible marker.

We'll skip the usual deprecation period because this was
explicitly a debugging aid that was never documented.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>