pretty: two phase conversion for non utf-8 commits

pretty: two phase conversion for non utf-8 commits

Always assume format_commit_item() takes an utf-8 string for string
handling simplicity (we can handle utf-8 strings, but can't with other
encodings).

If commit message is in non-utf8, or output encoding is not, then the
commit is first converted to utf-8, processed, then output converted
to output encoding. This of course only works with encodings that are
compatible with Unicode.

This also fixes the iso8859-1 test in t6006. It's supposed to create
an iso8859-1 commit, but the commit content in t6006 is in UTF-8.
t6006 is now converted back in UTF-8 (the downside is we can't put
utf-8 strings there anymore).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>