Speed up reflog pruning of unreachable commits

Speed up reflog pruning of unreachable commits

Instead of doing the (potentially very expensive) "in_merge_base()"
check for each commit that might be pruned if it is unreachable, do a
preparatory reachability graph of the commit space, so that the common
case of being reachable can be tested directly.

[ Cleaned up a bit and tweaked to actually work.  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>