Bugfix: Allow saving files/dirs on different fs with `--one-file-system`

restic now allows saving files/dirs on a different file system in a subdir
correctly even when `--one-file-system` is specified. 

The first thing the restic archiver code does is to build a tree of the target
files/directories. If it detects that a parent directory is already included
(e.g. `restic backup /foo /foo/bar/baz`), it'll ignore the latter argument.

Without `--one-file-system`, that's perfectly valid: If `/foo` is to be
archived, it will include `/foo/bar/baz`. But with `--one-file-system`,
`/foo/bar/baz` may reside on a different file system, so it won't be included
with `/foo`.

https://github.com/restic/restic/issues/1854
https://github.com/restic/restic/pull/1855