aawsome

I added some hints in the docu and realized that an extra options `--set-paths-from` would be also quite handy. This is now also added.

Also added some checks for the new flags in combination with stdin, analog to the checks of `--files-from*`.

About `--stdin`, we might think of making `--stdin-filename` an alias of `--set-path` and mark it deprecated. Then, for `--stdin` the only check we need is that only one path is...

@robvalca Thanks for your feedback! Is in your `/list.txt` only the line `/opt/this_is_a_test` or are there also the other files? As you write "This was always like this", I guess...

I would also like to leave the scope of this PR as it is. Can we open a new issue to discuss this changing of trees in the repo? I...

> Is this just awaiting review? From my side: yes

> I wanted to use this option to alter the snapshot paths and use the same paths between platforms (win/unix), but this patch keeps transforming the paths into [absolute](https://github.com/restic/restic/blob/fdbd65485e1d8b4f56159f7120a3277979921f4e/internal/restic/snapshot.go#L38). >...

> @aawsome I'm thinking it would be nice to use the same form of the noun `path` in both arguments, e.g. `--set-path` and `--set-path-from`, considering that (according to your first...

I think that it might be possible to extend restic such that it can give you the information which pack files are needed to fully restore a snapshot, see #2796...

@rptaylor While all of your point count into securing the repository, I think there are several issues: - Making your storage append-only (protecting all snapshots): This should IMO already work...