Added dirty-detection to WorkingTreeIterator

Added dirty-detection to WorkingTreeIterator

Added possibility to compare the current entry of a WorkingTreeIterator
to a given DirCacheEntry. This is done to detect whether an entry
in the index is dirty or not. 'Dirty' means that the file in the working tree
is different from what's in the index. Merge algorithms will make use of
this to detect conflicts.

Change-Id: I3ff847f4bf392553dcbd6ee236c6ca32a13eedeb
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>