rev-list: disable --use-bitmap-index when pruning commits

rev-list: disable --use-bitmap-index when pruning commits

The reachability bitmaps do not have enough information to
tell us which commits might have changed path "foo", so the
current code produces wrong answers for:

  git rev-list --use-bitmap-index --count HEAD -- foo

(it silently ignores the "foo" limiter). Instead, we should
fall back to doing a normal traversal (it is OK to fall
back rather than complain, because --use-bitmap-index is a
pure optimization, and might not kick in for other reasons,
such as there being no bitmaps in the repository).

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