These files were left behind, and made getKeysPresent find keys that were
not present. It would be expensive to make getKeysPresent check that the
actual key files are present (it just lists the directories). But that's not
needed if we just clean up the stale cache and mapping files.
To handle systems that were in direct mode and got switched back with stale
direct mode files, made cleanObjectLoc remove all files in the key's directory.
git annex unused will still list keys that are gone but for which the stale
direct mode files exists. To deal with that, made dropunused remove the key's
directory even if the key does not seem to be present.
Making the pre-commit hook look at git diff-index to find changed direct
mode files and update the mappings works pretty well.
One case where it does not work is when a file is git annex added, and then
git rmed, and then this is committed. That's a no-op commit, so the hook
probably doesn't even run, and it certianly never notices that the file
was deleted, so the mapping will still have the original filename in it.
For this and other reasons, it's important that the mappings still be
treated as possibly inconsistent.
Also, the assistant now allows the pre-commit hook to run when in direct
mode, so the mappings also get updated there.
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.
