sparse-revlog: handle nullrev in index_get_start

sparse-revlog: handle nullrev in index_get_start

The more generic index_get method handle nullrev fine, we apply the same logic
here.

--- a/mercurial/cext/revlog.c	Fri Dec 14 17:22:42 2018 +0100
+++ b/mercurial/cext/revlog.c	Fri Dec 14 17:23:41 2018 +0100
@@ -225,6 +225,9 @@
 static inline int64_t index_get_start(indexObject *self, Py_ssize_t rev)
 {
 	uint64_t offset;
+	if (rev == nullrev) {
+		return 0;
+	}
 	if (rev >= self->length) {
 		PyObject *tuple;
 		PyObject *pylong;