Expose beginning of iterator indication from AbstractTreeIterator
Callers like TreeWalk need to know if back(1) is going to be a valid
operation for a given AbstractTreeIterator before they try to make a
call to move the iterator backwards. The new method first() returns
true only if the iterator is already positioned on its first entry,
in which case a call to back(n) (for any n) is invalid.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>