Make unused variable optional in ChangesList::insertDiffHist

Make unused variable optional in ChangesList::insertDiffHist

$unpatrolled is unused in the method, so make it optional,
and then uses of it can be removed.

I realize it would do no harm to remove it completely, even
if a few (2 that I know of) extensions still pass the variable
to the method, but think this is nicer way.

Change-Id: Idbe6f00e9eb40db6a28de76fca0aea7c17b75656