lib/weakhash, lib/model, cmd/syncthing: Decide if to use weakhash on startup (fixes #3938)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3949

cmd/syncthing/mocked_config_test.go

@@ -9,6 +9,7 @@ package main
 import (
 	"github.com/syncthing/syncthing/lib/config"
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/util"
 )
 
 type mockedConfig struct {
@@ -24,7 +25,9 @@ func (c *mockedConfig) ListenAddresses() []string {
 }
 
 func (c *mockedConfig) RawCopy() config.Configuration {
-	return config.Configuration{}
+	cfg := config.Configuration{}
+	util.SetDefaults(&cfg.Options)
+	return cfg
 }
 
 func (c *mockedConfig) Options() config.OptionsConfiguration {