btrbk: add "prune" command

ChangeLog

@@ -6,6 +6,7 @@ btrbk-current
   * Add "resume" command, replacement for "-r, --resume-only" command
     line option (which is now deprecated).
   * Add "snapshot" command (close #150).
+  * Add "prune" command.
   * Add "--preserve-snapshots" and "--preserve-backups" options.
   * Change raw backup format (sidecar file instead of uuid in file).
   * Honor target_preserve for raw targets (delete raw targets).

btrbk

@@ -315,6 +315,7 @@ sub HELP_MESSAGE
   print STDERR "   dryrun                don't run btrfs commands; show what would be executed\n";
   print STDERR "   snapshot              run snapshot operations only\n";
   print STDERR "   resume                run backup operations, and delete snapshots\n";
+  print STDERR "   prune                 only delete snapshots and backups\n";
   print STDERR "   stats                 print snapshot/backup statistics\n";
   print STDERR "   list <subcommand>     available subcommands are:\n";
   print STDERR "      backups            all backups and corresponding snapshots\n";
@@ -4226,6 +4227,13 @@ MAIN:
     $args_allow_group = 1;
     @filter_args = @ARGV;
   }
+  elsif($command eq "prune") {
+    $action_run = 1;
+    $skip_snapshots = 1;
+    $skip_backups = 1;
+    $args_allow_group = 1;
+    @filter_args = @ARGV;
+  }
   elsif ($command eq "clean") {
     $action_clean = 1;
     @filter_args = @ARGV;

doc/btrbk.1

@@ -223,6 +223,17 @@ retention policy. Use in conjunction with \-p, \-\-preserve,
 backup and/or snapshot deletion (steps 3, 4).
 .RE
 .PP
+.B prune
+[filter...]
+.RS 4
+Prune snapshots and backups: skips snapshot and backup creation (steps
+1, 2), only deletes snapshots and backups in order to satisfy their
+configured retention policy. Useful for cleaning the disk after
+changing the retention policy. Use in conjunction with
+\-\-preserve\-backups, \-\-preserve\-snapshots if you want to skip
+backup or snapshot deletion (steps 3, 4).
+.RE
+.PP
 .B archive
 <source> <target>
 .I *experimental*