Allow --tag and --keep-tag to match untagged snapshots

doc/060_forget.rst

@@ -207,6 +207,8 @@ The ``forget`` command accepts the following parameters:
     boundaries and not relative to when you run the ``forget`` command. Weeks
     are Monday 00:00 -> Sunday 23:59, days 00:00 to 23:59, hours :00 to :59, etc.
 
+.. note:: Specifying ``--keep-tag ''`` will match untagged snapshots only.
+
 Multiple policies will be ORed together so as to be as inclusive as possible
 for keeping snapshots.
 
@@ -234,6 +236,13 @@ To only keep the last snapshot of all snapshots with both the tag ``foo`` and
 
    $ restic forget --tag foo,bar --keep-last 1
 
+To ensure only untagged snapshots are considered, specify the empty string '' as
+the tag.
+
+.. code-block:: console
+
+   $ restic forget --tag '' --keep-last 1
+
 All the ``--keep-*`` options above only count
 hours/days/weeks/months/years which have a snapshot, so those without a
 snapshot are ignored.

doc/manual_rest.rst

@@ -165,12 +165,10 @@ command does that:
     create exclusive lock for repository
     modified tags on 1 snapshots
 
-Note the snapshot ID has changed, so between each change we need to look
-up the new ID of the snapshot. But there is an even better way, the
-``tag`` command accepts ``--tag`` for a filter, so we can filter
-snapshots based on the tag we just added.
-
-So we can add and remove tags incrementally like this:
+Note the snapshot ID has changed, so between each change we need to look up the
+new ID of the snapshot. But there is an even better way - the ``tag`` command
+accepts a filter using the ``--tag`` option, so we can filter snapshots based
+on the tag we just added. This way we can add and remove tags incrementally:
 
 .. code-block:: console
 
@@ -189,6 +187,14 @@ So we can add and remove tags incrementally like this:
     $ restic -r /srv/restic-repo tag --tag NL --add SOMETHING
     no snapshots were modified
 
+To operate on untagged snapshots only, specify the empty string ``''`` as the
+filter value to ``--tag``. The following command will add the tag ``OTHER``
+to all untagged snapshots:
+
+.. code-block:: console
+
+    $ restic -r /srv/restic-repo tag --tag '' --add OTHER
+
 Under the hood
 --------------