Enhancement: Support `<snapshot>:<subfolder>` syntax to select subfolders

Commands like `diff` or `restore` always worked with the full snapshot. This
did not allow comparing only a specific subfolder or only restoring that folder
(`restore --include subfolder` filters the restored files, but still creates the
directories included in `subfolder`).

The commands `diff`, `dump`, `ls` and `restore` now support the
`<snapshot>:<subfolder>` syntax, where `snapshot` is the ID of a snapshot (or
the string `latest`) and `subfolder` is a path within the snapshot. The
commands will then only work with the specified path of the snapshot. The
`subfolder` must be a path to a folder as returned by `ls`. Two examples:

`restic restore -t target latest:/some/path`
`restic diff 12345678:/some/path 90abcef:/some/path`

For debugging purposes, the `cat` command now supports `cat tree
<snapshot>:<subfolder>` to return the directory metadata for the given
subfolder.

https://github.com/restic/restic/issues/3871
https://github.com/restic/restic/pull/4334
