revlog: clean up the node of all revision stripped in the C code

For some obscure reason, the loop cleaning up node was skipping the first
element… I cannot see a reason for it. The overall code is running fine
nevertheless because the node are also explicitly deleted from python.

We want to delete this explicit deletion, so we need to fix that code first.

This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.

Differential Revision: https://phab.mercurial-scm.org/D7320

mercurial/cext/revlog.c

@@ -2489,7 +2489,7 b' static int index_slice_del(indexObject *'
 		if (self->ntinitialized) {
 			Py_ssize_t i;
 
-			for (i = start + 1; i < self->length; i++) {
+			for (i = start; i < self->length; i++) {
 				const char *node = index_node_existing(self, i);
 				if (node == NULL)
 					return -1;