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