Browser: improve decision on when to split window for merger view.

Browser: improve decision on when to split window for merger view.

Now that we are tracking in_conflict in the broswer we "know" when
we are in a conflict and so can only display the split window then,
rather than keeping some hysteresis until we leave any changes behind.
This makes large jumps ('N' 'P') display better.
Also don't allow hunk headers to look like a 'wiggled' section, because
they aren't really.

Signed-off-by: NeilBrown <neilb@suse.de>