BitmapCalculator: Get the reachability bitmap of a commit

BitmapCalculator: Get the reachability bitmap of a commit

To make reachability checks with bitmaps, we need to get the
reachability bitmap of a commit, which is not always precalculated.

There is already a class returning such bitmap (BitmapWalker) but it
does too much unnecessary work: it calculates ALL reachable objects from
a commit (i.e. including trees and blobs), when for reachability the
commits are just enough.

Introduce BitmapCalculator to get the bitmap of a commit: either because
it is precalculated or generating it with a walk only over commits.

Change-Id: Ibb6c78affe9eeaf1fa362a06daf4fd2d91c1caea
Signed-off-by: Ivan Frade <ifrade@google.com>