Fix TreeWalk.idEqual when both trees are missing the path
The Javadoc of idEqual() says its simply a faster form of
getObjectId(nthA).equals(getObjectId(nthB)), but its code
didn't match that definition when both trees didn't exist
at the current path.
If a tree doesn't exist for the current path getObjectId() returns
ObjectId.zero(), indicating the "magic" 0{40} SHA-1 for the current
path. If both tree entries don't exist for the current path, we
should be doing a compare of ObjectId.zero() against ObjectId.zero(),
which must be true as the values are the same.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>