test-lib.sh/test_decode_color(): use octal not hex in awk script

test-lib.sh/test_decode_color(): use octal not hex in awk script

POSIX awk seems to explicitly not support hexadecimal escape sequences.

From http://pubs.opengroup.org/onlinepubs/009695399/:

   Regular expressions in awk have been extended somewhat...
   One sequence that is not supported is hexadecimal value escapes
   beginning with '\x'.

This affects the awk on IRIX 6.5, and causes t4015.56 to fail.
Use octal instead.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>