Adds DiffEntry.scan(TreeWalk, boolean) method

Adds DiffEntry.scan(TreeWalk, boolean) method

Adds method into DiffEntry class that allows to specify whether changed
trees are included in scanning result list. By default changed trees
aren't added, but in some cases having changed tree would be useful.

Also adds check for tree count in TreeWalk and when it is different from
two it will thrown an IllegalArgumentException.

This change is required by egit
I7ddb21e7ff54333dd6d7ace3209bbcf83da2b219

Change-Id: I5a680a73e1cffa18ade3402cc86008f46c1da1f1
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>