notify: ignore single-level ref name changes by default

notify: ignore single-level ref name changes by default

When using linked working trees with a Girocco repository (or even,
gasp, a non-bare repository), "single-level" refs may end up being
created in the repository such as "refs/stash".

These "single-level" refs are almost always meant to be local and
although a mirror clone will indeed fetch such refs, it's notable
that even a "mirror" push will not push them.

Changes to such refs are not generally something that should generate
any kind of notification.

Therefore ignore changes to any such "single-level" refs when
presented to the Girocco notification machinery unless the new
$Girocco::Config::notify_single_level preference has been explicitly
set to a true value.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>