Bugfix: Make `backup` and `tag` commands separate tags by comma

Running `restic backup --tag foo,bar` previously created snapshots with one
single tag containing a comma (`foo,bar`) instead of two tags (`foo`, `bar`).

Similarly, the `tag` command's `--set`, `--add` and `--remove` options would
treat `foo,bar` as one tag instead of two tags. This was inconsistent with
other commands and often unexpected when one intended `foo,bar` to mean two
tags.

To be consistent in all commands, restic now interprets `foo,bar` to mean two
separate tags (`foo` and `bar`) instead of one tag (`foo,bar`) everywhere,
including in the `backup` and `tag` commands.

NOTE: This change might result in unexpected behavior in cases where you use
the `forget` command and filter on tags like `foo,bar`. Snapshots previously
backed up with `--tag foo,bar` will still not match that filter, but snapshots
saved from now on will match that filter.

To replace `foo,bar` tags with `foo` and `bar` tags in old snapshots, you can
first generate a list of the relevant snapshots using a command like:

    restic snapshots --json --quiet | jq '.[] | select(contains({tags: ["foo,bar"]})) | .id'

and then use `restic tag --set foo --set bar snapshotID [...]` to set the new
tags. Please adjust the commands to include real tag names and any additional
tags, as well as the list of snapshots to process.

https://github.com/restic/restic/issues/2688
https://github.com/restic/restic/pull/2690
https://github.com/restic/restic/pull/3197
