Support for RevSort.BOUNDARY in ObjectWalk

Support for RevSort.BOUNDARY in ObjectWalk

When RevSort.BOUNDARY strategy in enabled, ObjectWalk now includes in
nextObjects() all objects associated with boundary commits (trees,
blobs) and all other objects explictly marked as  uninteresting
(boundary).

This behavior is something more than original C git-rev-list offers in
this matter - it is impossible to get such a behavior (to include all
boundary objects, not only commits, at output) directly from:
$ git-rev-list --objects-edge
Here, it is added for compactness - callers usually need also boundary
objects (e.g. for preparing thin-pack). If not, they can still easily
filter out such objects from nextObject() by checking for UNINTERESTING
flag or just use next() if interested only in commits.

Signed-off-by: Marek Zawirski <marek.zawirski@gmail.com>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>