The same way that [[git-annex-import]] has a `--duplicate` option,
there should be a way to reinject content without dropping the
original file.

My use case is that I had imported content in the wrong git-annex
repository, by mistake. Unaware of my mistake, I moved the files (now
symlinks) to the proper git-annex repo, where I added them and where
they (obviously) were broken. So I tried to fix that with reinject:

    $ git annex reinject --known ~/mp3/.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac
    reinject /home/anarcat/mp3/.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac 
    git-annex: /home/anarcat/mp3/.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac: rename: permission denied (Permission denied)
    failed
    (recording state in git...)
    git-annex: reinject: 1 failed

It *almost* worked but failed because it cannot rename the git-annex
file on the other side. But worse, copying the file by hand does not 
work either:

    $ LANG=C cp ~/mp3/.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac ../../../.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac
    cp: cannot create regular file '../../../.git/annex/objects/xJ/K5/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac/SHA256E-s16309242--41eb375ac760bf8542bae1ec2bcfce9269f78302820bcc88904e76bab473181c.flac': Permission denied

... obviously because the directory already exists and is not
writable. The workaround is to create those directories if missing
(`mkdir -p`) or change the mode if present (`chmod u+w`), then run
`git-annex-fsck` which will fix those modes.

Other than making reinject work against a fellow git-annex repository,
I think this would be useful to avoid deleting files which might
surprise users... I suggest using a logic similar to
[[git-annex-import]] for consistency reasons.

Thanks! -- [[anarcat]]