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>
(cherry picked from git.git commit 781262c5e7ad4a7813c528803117ed0d2e8c5172)
Signed-off-by: Fredrik Roubert <roubert@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>
Change-Id: I51fe2eb1eb879492a61e8e09c86ee34d049036c1