In sys.version and initial REPL message, list the source commit as "default"
    
    We create an artificial Git repo to apply patches.
    Python build system thinks that it's building from Git and embeds a bogus
    branch name into its version string:
    
    $ python3
    Python 3.9.0b5 (heads/master-dirty:24c7f56, Aug  3 2020, 00:00:00)
    [GCC 10.2.1 20200804 (Red Hat 10.2.1-2)] on linux
    Type "help", "copyright", "credits" or "license" for more information.
    
    Tell configure that Git is not installed. This reverts that part
    of sys.version to "default".