t5100: use ancient encoding syntax for backwards compatibility

t5100: use ancient encoding syntax for backwards compatibility

Some ancient platforms do not have an extensive list of alternate names for
character encodings.  For example, Solaris 7 does not know that ISO-8859-1
is the same as ISO8859-1.  Modern platforms do know this, so use the older
names.

The following conversions were performed:

    ISO-8859-1 --> ISO8859-1
    ISO-8859-2 --> ISO8859-2
    ISO-8859-8 --> ISO8859-8
    iso-2022-jp --> ISO-2022-JP

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>