Add tool "diff" for Linux and Unix

[shekharsorot]

Diff is used in Linux/Unix to compare two files, two directories or two streams.

In our case, Diff is used in XFStest since we need to compare *.full and *.out.full files for failed subtests to know what has went wrong.

TODO : Currently this tool is only for Linux and Unix, Windows alternative is FC which needs to be implemented.
Currently only "comparefiles" is implemented. as we come across requirements to compare directories and streams, we will add support for them.

        Args:
            src (PurePath): The source file or directory.
            dest (PurePath): The destination file or directory.
            cwd (Optional[PurePath]): The current working directory for the command.
            Defaults to None.
            sudo (bool): Whether to run the command with sudo. Defaults to False.
            timeout (int): The timeout for the command in seconds. Defaults to 600.
        Returns:
            str: The output of the diff command.

        Raises:
            AssertionError: If the exit code of the diff command is not 0 or 1.

0 means no changes, 1 means changes detected. 2 means general failure.

Edit: Due to a checksum issue, Github was treating init.py as new file. This has been fixed with a new commit and a new PR is being raised.

[squirrelsc]

@LiliDeng LGTM