Include reasons in json output of forget

This dumps the reasons as well as the list of keeps and removes with the
output from the forget command.

cmd/restic/cmd_forget.go

@@ -236,6 +236,8 @@ func runForget(opts ForgetOptions, gopts GlobalOptions, args []string) error {
 			}
 			addJSONSnapshots(&fg.Remove, remove)
 
+			fg.Reasons = reasons
+
 			jsonGroups = append(jsonGroups, &fg)
 
 			removeSnapshots += len(remove)
@@ -276,6 +278,7 @@ type ForgetGroup struct {
 	Paths   []string            `json:"paths"`
 	Keep    []Snapshot          `json:"keep"`
 	Remove  []Snapshot          `json:"remove"`
+	Reasons []restic.KeepReason `json:"reasons"`
 }
 
 func addJSONSnapshots(js *[]Snapshot, list restic.Snapshots) {