A merge may write files to the working tree. After a successful
merge one must fire a WorkingTreeModifiedEvent explicitly if
getModifiedFiles() is not empty.
Also, any touched files must be reported by the
WorkingTreeModifiedEvent fired by DirCacheCheckout.checkout().
Bug: 552636
Change-Id: I5fab8279ed8be8a4ae34cddfa726836b9277aea6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>