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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL