Enhancement: Add archive diff command

[LegoChicken]

Add a command which list the file difference between archives. Would give something like:
"- deleted_file"
"+ added_file"
"* changed_file"
Something like the obnam diff command. Syntax:
attic diff /data/myrepo.attic "archive label 1" "archive label 2"
"archive label 2" could be omitted and it would use the archive chronologically before "archive label 1".
When run an automated backup I email a summary to myself. Having this list is very useful as it shows whether something has been deleted when it shouldn't have been.

[dnnr]

You can already do this with any third-party tools of your own choice, using the FUSE mount feature in attic. I wouldn't suggest reinventing the wheel here.

[LegoChicken]

Although it could be done by a third party tool, it would be slow as to detect differences it would have read the files. Attic could just look at the metadata which would be much faster.

[dnnr]

I haven't checked if attic maybe even already does this, but the information you need for this could still be exposed by attic in the FUSE mount via hardlinks. Same inode numbers for the same file in two archives -> file hasn't changed. After all, attic should be well aware if two file paths reference the same content, and there's no need to hide that information in the mountpoint.

[LegoChicken]

I couldn't see a way of doing using existing attic commands. Although the information may be available via the FUSE mount, it would be a slower to obtain. This would be particularly the case for remote repositories. So I think it should be an internal command because attic would be able to use the metadata.

[dnnr]

There's no reason for such an internal command to be much faster (by any significant measure) than extracting the information from the FUSE mount. Granted, the FUSE mount could be implemented more efficiently than it is currently, but then I'd advise to work on this instead of introducing redundant features.

[LegoChicken]

I think this would not be a redundant feature. It is important for backup tool to be able to tell the user what it has done.

[anarcat]

this is similar to #158