hooks/pre-auto-gc-battery: allow gc to run on non-laptops

Desktops and servers tend to have no power sensor, thus on_ac_power returns 255 ("unknown"). Thus, let's take any answer other than 1 ("battery") as no contraindication to run gc. If that tool returns "unknown", there's no point in querying other sources as it already queried them, and is smarter than us (can handle multiple adapters). Reported by: Xin Li <delphij@google.com> Signed-off-by: Adam Borowski <kilobyte@angband.pl> Signed-off-by: Junio C Hamano <gitster@pobox.com>