Would it be possible to make `thin` work in locked mode? In other words, could locked mode use **hardlinks** instead of symlinks then `thin` is set?

Such a combined mode would have all the benefits of locked mode (protection, easy-to-spot missing files) and of direct mode (no duplication of files). It would also (partially) solve the problems discussed in [[devblog/day_601__v7_default/]] and [[todo/symlinks_for_not-present_unlocked_files/]].

Here is an example of how this could work from the user prospective:

```
$ mkdir foobar && cd foobar/ && git init . && git annex init foobar
$ echo "aaaa" > a && echo "bbbb" > b

$ git config annex.thin true
$ git annex add a b
[...] ok
$ git annex sync
[...] ok
$ ls
a  b  # note: no symlinks

$ echo "foo" > a
bash: a: Permission denied

$ git annex drop --force a
$ ls
a@  b # note: symlink for missing file
```
