[PATCH 1 of 4 V2] revlog: introduce a constant for nullrev in `revlog.c`

[Yuya Nishihara]

On Sat, 15 Dec 2018 15:10:53 +0000, Boris Feld wrote:
> # HG changeset patch
> # User Boris Feld <boris.feld at octobus.net>
> # Date 1544804562 -3600
> #      Fri Dec 14 17:22:42 2018 +0100
> # Node ID c1e47daaab82e7d9340b0bd179d022fdd21062fc
> # Parent  2f14d1bbc9a7a142b421285c0708320b46be7a56
> # EXP-Topic sparse-followup
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r c1e47daaab82
> revlog: introduce a constant for nullrev in `revlog.c`

Queued, thanks.

> +static const Py_ssize_t nullrev = -1;

It's unclear whether nullrev should be int or ssize_t because our codebase
is so inconsistent about the type of the revision numbers, but this is
a good start.