reversion: deal with a git regression when the tree is clean

Some time between git-1.7.3 and git-1.7.10 `git show --quiet`
stopped working. Apparently the official way to suppress
the diff output is now with the -s option. Neither of these
options is documented in the `git show` man page; `git diff`
documents the --quiet option for this purpose and `git log`
documents the -s option.

See also http://thread.gmane.org/gmane.comp.version-control.git/174665

Reported-by: Bruce Korb <bruce.korb@gmail.com>