Use 'reused' bitmap to filter walk during bitmap selection

Use 'reused' bitmap to filter walk during bitmap selection

When building fullBitmap in order to determine which ancestor chain to
add this commit to, we were excluding the ancestors of reusedCommits
using markUninteresting.  This use of markUninteresting is a bit
wasteful because we already have a bitmap indicating exactly which
commits should be excluded (which can save some walking).  Use it.

A separate commit will remove the now-redundant markUninteresting
call.

No behavior change intended (except for performance improvement).

Change-Id: I1112641852d72aa05c9a8bd08a552c70342ccedb