gitweb: apply fallback encoding before highlight

Some multi-byte character encodings (such as Shift_JIS and GBK) have
characters whose final bytes is an ASCII '\' (0x5c), and they
will be displayed as funny-characters even if $fallback_encoding is
correct.  This is because `highlight` command always expects UTF-8
encoded strings from STDIN.

    $ echo 'my $v = "申";' | highlight --syntax perl | w3m -T text/html -dump
    my $v = "申";

    $ echo 'my $v = "申";' | iconv -f UTF-8 -t Shift_JIS | highlight \
        --syntax perl | iconv -f Shift_JIS -t UTF-8 | w3m -T text/html -dump

    iconv: (stdin):9:135: cannot convert
    my $v = "

This patch prepare git blob objects to be encoded into UTF-8 before
highlighting in the manner of `to_utf8` subroutine.

Signed-off-by: Shin Kojima <shin@kojima.org>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/gitweb.perl

@@ -3935,6 +3935,9 @@ sub run_highlighter {
 
 	close $fd;
 	open $fd, quote_command(git_cmd(), "cat-file", "blob", $hash)." | ".
+	          quote_command($^X, '-CO', '-MEncode=decode,FB_DEFAULT', '-pse',
+	            '$_ = decode($fe, $_, FB_DEFAULT) if !utf8::decode($_);',
+	            '--', "-fe=$fallback_encoding")." | ".
 	          quote_command($highlight_bin).
 	          " --replace-tabs=8 --fragment --syntax $syntax |"
 		or die_error(500, "Couldn't open file or run syntax highlighter");