remote show: do not show symbolic refs

remote show: do not show symbolic refs

For symbolic refs, a sane notion of being "stale" is that the ref
they point to no longer exists.  Since this is checked already,
"remote show" does not need to show them at all.

Incidentally, this fixes the issue that "HEAD" was shown as a
stale ref by "remote show" in a freshly cloned repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>