CodingGuidelines: use octal escapes, not hex

Extend the shell-scripting section of CodingGuidelines to suggest octal
escape sequences (e.g. "\302\242") over hexadecimal (e.g. "\xc2\xa2")
since the latter can be a source of portability problems.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -188,6 +188,10 @@ For shell scripts specifically (not exhaustive):
    hopefully nobody starts using "local" before they are reimplemented
    in C ;-)
 
+ - Use octal escape sequences (e.g. "\302\242"), not hexadecimal (e.g.
+   "\xc2\xa2") in printf format strings, since hexadecimal escape
+   sequences are not portable.
+
 
 For C programs: