gitweb: Fix handling of non-ASCII characters in inserted HTML files

gitweb: Fix handling of non-ASCII characters in inserted HTML files

Use new insert_file() subroutine to insert HTML chunks from external
files: $site_header, $home_text (by default indextext.html),
$site_footer, and $projectroot/$project/REAME.html.

All non-ASCII chars of those files will be broken by Perl IO layer
without decoding to utf8, so insert_file() does to_utf8() on each
printed line; alternate solution would be to open those files with
"binmode $fh, ':utf8'", or even all files with "use open qw(:std :utf8)".

Note that inserting README.html lost one of checks for simplicity.

Noticed-by: Tatsuki Sugiura <sugi@nemui.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>