[PATCH 4 of 4 V2] sparse-revlog: protect C code against delta chain including nullrev

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1544804741 -3600
#      Fri Dec 14 17:25:41 2018 +0100
# Node ID e7d33dc28696a1b2ee951cb82fe2cd611037afc8
# Parent  36c68746763d6b93b00c19387e79bd9cb623da72
# EXP-Topic sparse-followup
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r e7d33dc28696
sparse-revlog: protect C code against delta chain including nullrev

For unclear reasons, some repositories include nullrev (-1). Re-computing
delta for such repo remove nullrev from all chain, so some older versions have
been creating them.

This currently raise an IndexError with the new C code doing chain slicing as
it expect all item to be positive.

Both python and C code for reading delta chain preserve nullrev, and the Python
code for chain slicing handle the case fine. So we take the safe route and make
the new C code works fine in that case.

diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
--- a/mercurial/cext/revlog.c
+++ b/mercurial/cext/revlog.c
@@ -1195,7 +1195,7 @@ static PyObject *index_slicechunktodensi
 		if (revnum == -1 && PyErr_Occurred()) {
 			goto bail;
 		}
-		if (revnum < 0 || revnum >= idxlen) {
+		if (revnum < -1 || revnum >= idxlen) {
 			PyErr_Format(PyExc_IndexError,
 			             "index out of range: %zd", revnum);
 			goto bail;