gitweb: Strip non-printable characters from syntax highlighter output

gitweb: Strip non-printable characters from syntax highlighter output

The current code, as is, passes control characters, such as form-feed
(^L) to highlight which then passes it through to the browser.  User
agents (web browsers) that support 'application/xhtml+xml' usually
require that web pages declared as XHTML and with this mimetype are
well-formed XML.  Unescaped control characters cannot appear within a
contents of a valid XML document.

This will cause the browser to display one of the following warnings:

* Safari v5.1 (6534.50) & Google Chrome v13.0.782.112:

   This page contains the following errors:

   error on line 657 at column 38: PCDATA invalid Char value 12
   Below is a rendering of the page up to the first error.

* Mozilla Firefox 3.6.19 & Mozilla Firefox 5.0:

   XML Parsing Error: not well-formed
   Location:
   http://path/to/git/repo/blah/blah

Both errors were generated by gitweb.perl v1.7.3.4 w/ highlight 2.7
using arch/ia64/kernel/unwind.c from the Linux kernel.

When syntax highlighter is not used, control characters are replaced
by esc_html(), but with syntax highlighter they were passed through to
browser (to_utf8() doesn't remove control characters).

Introduce sanitize() subroutine which strips forbidden characters, but
does not perform HTML escaping, and use it in git_blob() to sanitize
syntax highlighter output for XHTML.

Note that excluding "\t" (U+0009), "\n" (U+000A) and "\r" (U+000D) is
not strictly necessary, atleast for currently the only callsite: "\t"
tabs are replaced by spaces by untabify(), "\n" is stripped from each
line before processing it, and replacing "\r" could be considered
improvement.

Originally-by: Christopher M. Fuhrman <cfuhrman@panix.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>