revlog: use literal -1 instead of variable that always has that value

We were setting "ntrev" to "rev", but "rev" was always -1 at the end
of the loop, so it's clearer to use a literal -1.

Differential Revision: https://phab.mercurial-scm.org/D3498

mercurial/cext/revlog.c

@@ -1243,7 +1243,7 b' static int nt_populate(indexObject *self'
 			if (nt_insert(self, n, rev) == -1)
 				return -1;
 		}
-		self->ntrev = rev;
+		self->ntrev = -1;
 	}
 	return 0;
 }