gitweb: Convert generated contents to utf8 in commitdiff_plain

gitweb: Convert generated contents to utf8 in commitdiff_plain

If the commit message, or commit author contains non-ascii, it must be
converted from Perl internal representation to utf-8, to follow what
got declared in HTTP header.  Use to_utf8() to do the conversion.

This necessarily replaces here-doc with "print" statements.

Signed-off-by: Yasushi SHOJI <redacted>
Acked-by: İsmail Dönmez <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>