db/simple/Directory: add method IsReallyAFile()

src/db/plugins/simple/Directory.hxx

@@ -111,6 +111,15 @@ public:
 		return new Directory(std::string(), nullptr);
 	}
 
+	/**
+	 * Is this really a regular file which is being treated like a
+	 * directory?
+	 */
+	bool IsReallyAFile() const noexcept {
+		return device == DEVICE_INARCHIVE ||
+			device == DEVICE_CONTAINER;
+	}
+
 	bool IsMount() const noexcept {
 		return mounted_database != nullptr;
 	}

src/db/update/UpdateIO.cxx

@@ -59,8 +59,7 @@ DirectoryExists(Storage &storage, const Directory &directory) noexcept
 		return false;
 	}
 
-	return directory.device == DEVICE_INARCHIVE ||
-		directory.device == DEVICE_CONTAINER
+	return directory.IsReallyAFile()
 		? info.IsRegular()
 		: info.IsDirectory();
 }