The idea, not yet done, is to use this to detect when a file
has an old change time, and avoid expensive restaging of the file.
If git-annex watch keeps track of the last time it finished a full scan,
then any symlink that is older than that time must have been scanned
before, so need not be added. (Relying on moving, copying, etc of a file
all updating its change time.)
Anyway, this info is available for free since inotify already checks it,
so it might as well make it available.
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