index: drop support for negative indexes into the index

I want index[i] to work for any valid revnum including -1 (as it
already does), and I also want len(index) to be the number of
revisions in the index (not counting the null revision), so it cannot
also support negative revision numbers other than -1 for nullid.

I didn't bother removing support for it from revlog v0.

Differential Revision: https://phab.mercurial-scm.org/D4019

mercurial/cext/revlog.c

@@ -163,9 +163,6 b' static PyObject *index_get(indexObject *'
 		return nullentry;
 	}
 
-	if (pos < 0)
-		pos += length;
-
 	if (pos < 0 || pos >= length) {
 		PyErr_SetString(PyExc_IndexError, "revlog index out of range");
 		return NULL;

mercurial/pure/parsers.py

@@ -48,8 +48,6 b' class BaseIndexObject(object):'
     def _fix_index(self, i):
         if not isinstance(i, int):
             raise TypeError("expecting int indexes")
-        if i < 0:
-            i = len(self) + i
         if i < 0 or i >= len(self):
             raise IndexError
         return i