Example case:
Display file written in iso-8859-1 with current HGENCODING utf-8.
At the moment only an Error page appears because pygmentize
chokes on the replacement chars.
Alternatives:
1) Turn off highlighting and avoid UnicodeDecodeError
for files that are not in HGENCODING.
2) [this patch] use util.tolocal to display these files.
Alternative 2) seems ok, as this only concerns display and
readability.
See also: c5f1a58b8b9a, apparently put aside during refactor of
highlight.
Add test for UnicodeDecodeError with iso-8859-1 file contents.
For non-html mimetypes it doesn't make much sense. This also fixes the
issue that highlight unconditionally adds a <link/> tag for its CSS to
the template's header (which is pointless in text/plain output).
