lib/config: Retain slash at end of path after expanding ~ (fixes #2782)

The various path cleaning operations done in in cleanedPath() removes
it, so we make sure it's added again at the end. This makes adding the
slash in prepare() unnecessary, but keep it anyway for display purposes
(people looking at the config).

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3342

lib/config/folderconfiguration.go

@@ -168,6 +168,12 @@ func (f *FolderConfiguration) cleanedPath() string {
 		return `\\?\` + cleaned
 	}
 
+	// If we're not on Windows, we want the path to end with a slash to
+	// penetrate symlinks. On Windows, paths must not end with a slash.
+	if runtime.GOOS != "windows" && cleaned[len(cleaned)-1] != filepath.Separator {
+		cleaned = cleaned + string(filepath.Separator)
+	}
+
 	return cleaned
 }