[PATCH 02 of 10 V2] revlog: fix pure python slicing code when chain contains nullrev

[Yuya Nishihara]

On Fri, 28 Dec 2018 00:24:48 +0100, Boris FELD wrote:
> > Can you turn these tests into a unittest to cover both pure and C
> > implementations?
> Turning this specific test is a bit more complicated than we would like
> as we are taking advantage of _testrevlog flexibility to produce a
> simple but clear test.
> 
> However, we've already added some unittest using real revlog in patches
> you have already taken. Is that enough for you?

The unittest you wrote is nice, thanks, but it's even nicer if we had
a single test set that covers both pure and cext. It'll work as a source
of truth. At this point, we have to carefully compare both implementations
to be sure they are identical.