t6006 (rev-list-format): quote format strings to avoid error on some shells

Added in 0a144b3 (t4205, t6006: add failing tests for the case when
i18n.logOutputEncoding is set, 2014-05-19) tests give no error
(somehow) with Bash as /bin/sh but fail for some other shells.

Quote format strings to avoid errors.

Signed-off-by: Alexey Shumkin <Alex.Crezoff@gmail.com>
Suggested-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>