Even though latin-1 is still seen in e-mail headers, some platforms
only install ISO-8859-1.  "iconv -f ISO-8859-1" succeeds, while
"iconv -f latin-1" fails on such a system.

Using the same fallback_encoding() mechanism factored out in the
previous step, teach ourselves that "ISO-8859-1" has a better chance
of being accepted than "latin-1".

Signed-off-by: Junio C Hamano <gitster@pobox.com>