forget: refuse deleting the last snapshot in a snapshot group

`--keep-tag invalid-tag` was previously able to wipe all snapshots in a
repository. As a user specified a `--keep-*` option this is likely
unintentional. This forbid deleting all snapshot if a `--keep-*` option
was specified to prevent data loss. (Not specifying such an option
currently also causes the command to abort)

cmd/restic/cmd_forget.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
 	"strconv"
 
@@ -248,6 +249,10 @@ func runForget(ctx context.Context, opts ForgetOptions, pruneOptions PruneOption
 
 				keep, remove, reasons := restic.ApplyPolicy(snapshotGroup, policy)
 
+				if !policy.Empty() && len(keep) == 0 {
+					return fmt.Errorf("refusing to delete last snapshot of snapshot group %v", key)
+				}
+
 				if len(keep) != 0 && !gopts.Quiet && !gopts.JSON {
 					printer.P("keep %d snapshots:\n", len(keep))
 					PrintSnapshots(globalOptions.stdout, keep, reasons, opts.Compact)