gitweb: Use character or octal escape codes (and add span.cntrl) in esc_path

gitweb: Use character or octal escape codes (and add span.cntrl) in esc_path

Instead of simply hiding control characters in esc_path by replacing
them with '?', use Character Escape Codes (CEC) i.e. alphabetic
backslash sequences like those found in C programming language and
many other languages influenced by it, such as Java and Perl.  If
control characted doesn't have corresponding character escape code,
use octal char sequence to escape it.

Alternatively, controls can be replaced with Unicode Control
Pictures U+2400 - U+243F (9216 - 9279), the Unicode characters
reserved for representing control characters when it is
necessary to print or display them.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>