Following the list discussion, define two functions, quote_c_style and
unquote_c_style, to help adopting the proposed way for quoting funny
pathname letters for GNU patch. The rule is described in:
http://marc.theaimsgroup.com/?l=git&m=
112927316408690&w=2
Currently we do not support the leading '!', but we probably should
barf upon seeing it. Rule B4. is interpreted to require always 3
octal digits in \XYZ notation.
Signed-off-by: Junio C Hamano <junkio@cox.net>