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

lib/config/config.go

@@ -332,6 +332,19 @@ func convertV17V18(cfg *Configuration) {
 	cfg.Version = 18
 }
 
+func convertV16V17(cfg *Configuration) {
+	for i := range cfg.Folders {
+		cfg.Folders[i].Fsync = true
+	}
+
+	cfg.Version = 17
+}
+
+func convertV15V16(cfg *Configuration) {
+	// Triggers a database tweak
+	cfg.Version = 16
+}
+
 func convertV14V15(cfg *Configuration) {
 	// Undo v0.13.0 broken migration
 
@@ -347,19 +360,6 @@ func convertV14V15(cfg *Configuration) {
 	cfg.Version = 15
 }
 
-func convertV15V16(cfg *Configuration) {
-	// Triggers a database tweak
-	cfg.Version = 16
-}
-
-func convertV16V17(cfg *Configuration) {
-	for i := range cfg.Folders {
-		cfg.Folders[i].Fsync = true
-	}
-
-	cfg.Version = 17
-}
-
 func convertV13V14(cfg *Configuration) {
 	// Not using the ignore cache is the new default. Disable it on existing
 	// configurations.