Fix '\%o' for printf from coreutils

The printf utility provided by coreutils when interpreting '\%o' format
does not recognize %o as formatting directive. For example
printf '\%o 0 returns \%o and warning: ignoring excess arguments,
starting with ‘0’, which results in failed tests in
t5309-pack-delta-cycles.sh. In most shells the test ends with success as
the printf is a builtin utility.

Fix it by using '\\%o' which is interpreted consistently in all versions
of printf.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>