magit-insert-wazzup-cherries: use magit-git-wash, add to parent
keep delayed text insertion from messing up section boundaries
magit-git-wash: pass arguments to washer too

Pass the arguments to git that are used to insert the text also to the
function that washes that output.  It helps those functions to know what
exactly they are supposed to wash.

In one case (washing cherries) we start taking advantage of this now.
magit-wash-diff: fix to bugs related to unmerged files

Make sure we return non-nil, otherwise subsequent diffs would not be
washed (including, but not limited to the diff about the same file as
the "unmerged" line).

In the status buffer, only remove the duplicated "unmerged" line, not
the original too.