Tests incompatible with Git<2.28

[jedie]

I tried to create a dev setup to contribute. (Sadly https://github.com/akaihola/darker/blob/master/CONTRIBUTING.rst doesn't contain how to get started)

Running tests i get this error:

    def check_call(*popenargs, **kwargs):
        """Run command with arguments.  Wait for command to complete.  If
        the exit code was zero then return, otherwise raise
        CalledProcessError.  The CalledProcessError object will have the
        return code in the returncode attribute.
    
        The arguments are the same as for the call function.  Example:
    
        check_call(["ls", "-l"])
        """
        retcode = call(*popenargs, **kwargs)
        if retcode:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = popenargs[0]
>           raise CalledProcessError(retcode, cmd)
E           subprocess.CalledProcessError: Command '['git', 'init', '--initial-branch=master']' returned non-zero exit status 129.

/usr/lib/python3.9/subprocess.py:373: CalledProcessError
---------------------------- Captured stderr setup -----------------------------
error: unknown option `initial-branch=master'
usage: git init [-q | --quiet] [--bare] [--template=<template-directory>] [--shared[=<permissions>]] [<directory>]

    --template <template-directory>
                          directory from which templates will be used
    --bare                create a bare repository
    --shared[=<permissions>]
                          specify that the git repository is to be shared amongst several users
    -q, --quiet           be quiet
    --separate-git-dir <gitdir>
                          separate git dir from working tree

I have git --version is: 2.25.1

[akaihola]

Yes, seems that the git init --initial-branch=... option only appeared in Git 2.28.

I think we should parse the output of git --version and leave the --initial-branch=master option out if the installed Git version is older than 2.28.