Allow specifing a snapshot and a relative localpath for `rustic diff`

[gyscos]

Currently, rustic diff has the following usage:

Usage: rustic diff [OPTIONS] <SNAPSHOT1[:PATH1]> <SNAPSHOT2[:PATH2]|PATH2>

I would like to be able to simply run:

  rustic diff $snapshot_id:$file_path

Instead, I currently have to run:

  rustic diff $snapshot_id:$file_path $file_path

Rustic already infers PATH2 to be PATH1 if a snapshot ID is given, and if no snapshot ID is given, it knows to look for local paths. I would like to combine both: if a single argument is given (no snapshot ID, no PATH2), defaults to PATH1 locally.

For the same use-case of getting the diff between a snapshot and a local file, it might also be nice to be able to specify a snapshot ID and a relative path (instead of an absolute path for PATH1 above):

    rustic diff $snapshot_id ./local_file

Or even

    rustic diff $snapshot_id local_file

(In that last one, rustic currently things local_file is a snapshot ID.)

It would then internally get the absolute path of the given file, and compare the snapshot version with that local file.

Thank you for this great tool!