The most common way for a mapping to be stale is when a file was deleted,
or renamed. Nothing updates the mappings for deletions yet.
But they can also become stale in other ways. For example a file can
be modified.
So, the mapping is not trusted to be consistent. When we get a key,
only replace symlinks that still point to that key with its content.
When we drop a key, only put back symlinks for files that still have
the direct mode content.
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