store: only add new entries to the fncache file
Newly added fncache entries were not added to the in-memory cache,
making it possible for 'hg convert' to cause duplicates in
.hg/store/fncache.
Duplicates in the fncache file are harmless, but excessive numbers
of duplicates from large converted repositories may slow down
execution speed considerably.
Updated test output of test-convert-baz after
f2618cacb485