BitmappedReachabilityChecker: Reachability check using bitmaps

BitmappedReachabilityChecker: Reachability check using bitmaps

The "basic" reachability check walks the graph starting from the tips
marking things as "uninteresting". If the target commit is marked as
"uninteresting" it was reached; it is reachable from those tips.

This requires a lot of walking and can be solved directly with bitmaps.
Most of the time the bitmaps are already calculated or a short walk
away.

This should improve the performance of reachability checks, for example
in Gitiles.

Change-Id: I83d33271f58d95d2dc9ed151967b3eda513c99f7
Signed-off-by: Ivan Frade <ifrade@google.com>