avoid issues with i18n in tests

On Unix/Linux, git uses the LANGUAGE environment variable to localize messag

This might need further tweaking on msysgit/Win32.
LC_ALL=C is a safer way to ensure the absence of localization

https://www.gnu.org/software/gettext/manual/gettext.html#The-LANGUAGE-variable

    GNU gettext gives preference to LANGUAGE over LC_ALL and LANG for
    the purpose of message handling, but you still need to have LANG
    (or LC_ALL) set to the primary language; this is required by other
    parts of the system libraries.

    [...]

    Note: The variable LANGUAGE is ignored if the locale is set to
    ‘C’. In other words, you have to first enable localization,
    by setting LANG (or LC_ALL) to a value other than ‘C’, before
    you can use a language priority list through the LANGUAGE variable.
tutorial: prevent localization of the output of Git commands

Thanks to Lars Dieckow (DAXIM) for a test report obtained from
testing Git::Repository under the de_DE.UTF-8 locale.
document all the ways to setup LC_ALL