The checkout subcommand replaces the symlink that normally points at a
file's content, with a copy of the file. Once you've checked a file out,
you can edit it, and `git commit` it. On commit, git-annex will detect
if the file has been changed, and if it has, `add` its content to the
annex.

> Internally, this will need to store the original symlink to the file, in 
> `.git/annex/checkedout/$filename`.
> 
> * git-annex uncheckout moves that back
> * git-annex pre-commit hook checks each file being committed to see if
>   it has a symlink there, and if so, removes the symlink and adds the new
>   content to the annex. 
> 
> And it seems the file content should be copied, not moved or hard linked:
> 
> * Makes sure other annexes can find it if transferring it from
>   this annex.
> * Ensures it's always available for uncheckout.
> * Avoids the last copy of a file's content being lost when
>   the checked out file is modified.

[[done]]
