Old data isn't listed as unused after migrating backends:

    #!/bin/bash
    
    BASE=/tmp/migrate-bug-2
    set -x
    chmod -R +w $BASE
    rm -rf $BASE
    mkdir -p $BASE
    cd $BASE
    
    # create annex
    git init .
    git annex init
    
    # make a big (sparse) file and add it
    dd if=/dev/zero of=bigfile bs=1 count=0 seek=1G
    git annex add --backend WORM bigfile
    git commit -m 'added bigfile'
    
    # migrate it
    git annex migrate --backend SHA256 bigfile
    
    # status shows 2 keys taking up 2G
    git annex status
    
    # but nothing is unused
    git annex unused

Output:

    ++ git annex status
    supported backends: SHA256 SHA1 SHA512 SHA224 SHA384 SHA256E SHA1E SHA512E SHA224E SHA384E WORM URL
    supported remote types: git S3 bup directory rsync web hook
    known repositories: 
            ede95a82-1166-11e1-a475-475d55eb0f8f -- here
    local annex keys: 2
    local annex size: 2 gigabytes
    visible annex keys: 1
    visible annex size: 1 gigabyte
    backend usage: 
            WORM: 1
            SHA256: 1
    ++ git annex unused
    unused . (checking for unused data...) (checking master...) ok

The two files are hardlinked, so it's not taking up extra space, but it would be nice to be able to remove the old keys.

> `git annex unused` checks the content of all branches, and assumes that,
> when a branch contains a file that points to a key, that key is still 
> used. In this case, the migration has staged a change to the file,
> but it is not yet committed, so when it checks the master branch, it
> still finds a file referring to the old key. 
> 
> So, slightly surprising, but not a bug. --[[Joey]] [[done]]

>> Thanks for the explanation.  In my real repository, it was a bit trickier:
>> the migration was commited to `master`, but other *remote* branches still
>> referenced those keys.  I was just doing a `git pull` from a central repo, but
>> needed a `git remote update` to remove those references from `remotes/foo/master` too.
>> --Jim

>>> I have considered making unused ignore remote tracking branches. 
>>> On the one hand, it can be a little bit confusing, and those branches
>>> can be out of date. On the other hand, it can be useful to know you're
>>> not dropping anything that some remote might still refer to. --[[Joey]] 
