Объединение изменений PHP с diff / patch

I have three PHP files:

• 1.php - Version 1.0 of OSS software (vanilla)
• 1a.php - Version 1.0 of OSS software (with customisations)
• 2.php - Version 1.1 of OSS software (vanilla)

I would like to use diff/patch (or similar) to create a single patch file that will merge the modifications I made to version 1.0 with the upstream changes made in version 1.1. What is the best way to achieve this?

Many thanks.