.DIFF File Extension
What is a DIFF file?
Patch file created by Mercurial, a control management tool for developers; contains a record of the differences between two versions of the same text or source file; enables updating of older text files to newer version text files; accessed by developers that are making changes to different versions of the same file; also known as .PATCH files.
The "patch" command searches for the line that starts with the string diff -, once it finds that string, it knows that that's the beginning of the patch content.
Mercurial is mostly written in the Python and C languages.