Specifying `/` casts a shadow on other `SOURCE`s

[endreszabo]

Coming from restic. With restic, I could back up three filesystems using the following command:

# restic -r 'rest:http://nas:8488/hostname' backup --one-file-system /boot /mailstore /

All three filesystems are getting backed up this way.

Using the very same command line with rustic only backs up the / filesystem, completely ignoring both /boot and /mailstore:

[INFO] starting to backup /...
[00:00:09] backing up...                  ████████████████████████████████████████   3.12 GiB/3.12 GiB   337.39 MiB/s (ETA -)                                   Files:       21 new, 9 changed, 57676 unchanged
Dirs:        23 new, 23 changed, 4802 unchanged
Added to the repo: 31.6 MiB (raw: 36.8 MiB)
processed 57706 files, 3.1 GiB
snapshot ef6f505f successfully saved.
[INFO] backup of / done

When I remove / from the SOURCEs, both /boot and /mailstore filesystems are getting backed up, as expected:

[INFO] starting to backup /boot,/mailstore...
[00:05:48] backing up...                  ████████████████████████████████████████  18.50 GiB/18.50 GiB  54.40 MiB/s  (ETA -)                                   Files:       218741 new, 0 changed, 0 unchanged
Dirs:        639 new, 0 changed, 0 unchanged
Added to the repo: 394.7 MiB (raw: 1.2 GiB)
processed 218741 files, 18.5 GiB
snapshot 044d01d5 successfully saved.
[INFO] backup of /boot,/mailstore done

This is a major difference in behaviour for which I couldn't find any mention among the github issues or in the docs.