The checker is creating a new bitmap per branch leading to excessive
memory consumption. For the reachability check one bitmap with the
reachability of all branches aggregated is enough.
Build the reachability bitmap with a filter. The filter itself uses it
to emit only commits not reached before and the caller to check what
targets have been reached already.
BitmapCalculator is not required anymore.
Change-Id: Ic5c62f77fe0f188913215b7eaa51d849a9aae6a5
Signed-off-by: Ivan Frade <ifrade@google.com>