patman: fix gitutil for decorations

The git config parameter log.decorate is quite useful when working with git.
Patman, however can not handle the decorated output when parsing the commit.
To prevent this use the '--no-decorate' switch for git-log.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Simon Glass <sjg@chromium.org>