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
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