lib/config, lib/model: Make sure to hide our special files (fixes #4382)

The folder marker conversion forgot to hide the .stfolder. This adds
that, for those who have not yet been converted.

Also adds Hide() calls to the folder start, to mend historical
unhidedness. (I'm sure this will upset someone who is manually managing
their .stignores in the other direction...)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4384

lib/model/model.go

@@ -253,7 +253,14 @@ func (m *Model) startFolderLocked(folder string) config.FolderType {
 		}
 	}
 
-	p := folderFactory(m, cfg, ver, fs.MtimeFS())
+	ffs := fs.MtimeFS()
+
+	// These are our metadata files, and they should always be hidden.
+	ffs.Hide(".stfolder")
+	ffs.Hide(".stversions")
+	ffs.Hide(".stignore")
+
+	p := folderFactory(m, cfg, ver, ffs)
 	m.folderRunners[folder] = p
 
 	m.warnAboutOverwritingProtectedFiles(folder)