Bugfix: overlap and f?(cat|sep)

The pretty source code currently contains two TODOs:

```
-- XXX: TODO: PRETTY: Used to use True here (but GHC used False...)
nilAboveNest False k (reduceDoc (vcat ys))
```

```
-- XXX: TODO: PRETTY: Used to use True here (but GHC used False...)
`mkUnion` nilAboveNest False k (fill g (y:ys))
```

I think we should go back to using True. From https://mail.haskell.org/pipermail/libraries/2008-June/009991.html (commit 1e50748beaa4bd2281d323b18ea51c786bba04a1):

```
2) Bugfix: overlap and f?(cat|sep)

The specification for cat/sep:
* oneLiner (hcat/hsep ps)
`union`
vcat ps [*]

But currently cat, sep, fcat and fsep attempt to overlap the second
line with the first one, i.e. they use
`foldr ($$) empty ps' instead of `foldr ($+$) empty ps' [*]. I assume
this is a mistake.

This bug can lead to situations, where the line in the right argument
of Union is actually longer:

> prettyDoc$ cat [ text "a", nest 2 ( text "b") ]
>> text "a"; union
>> (text "b"; empty)
>> (nilabove; nest 1; text "b"; empty)

> renderStyle (Style PageMode 1 1) $ cat [ text "a", nest 2 ( text
"b") ]
>> "a b"

In the implementation, we call `nilAbove False' instead of `nilAbove
True' (see patch).
```