revlog: update pure nodecache start lookup offset on insertion

test-storage.py is failing in pure builds because the population of the node
cache isn't accounting for the new starting offset after a revlog insertion.

This commit updates the node cache start offset to account for insertions.

I'm not 100% convinced this is the ideal solution. But it works and seems
correct.

Differential Revision: https://phab.mercurial-scm.org/D4996

mercurial/revlog.py

@@ -1937,6 +1937,11 b' class revlog(object):'
         self.index.append(e)
         self.nodemap[node] = curr
 
+        # Reset the pure node cache start lookup offset to account for new
+        # revision.
+        if self._nodepos is not None:
+            self._nodepos = curr
+
         entry = self._io.packentry(e, self.node, self.version, curr)
         self._writeentry(transaction, ifh, dfh, entry, deltainfo.data,
                          link, offset)