Merge branch 'ab/pre-auto-gc-battery'

A sample auto-gc hook (in contrib/) to skip auto-gc while on
battery has been updated to almost always allow running auto-gc
unless on_ac_power command is absolutely sure that we are on
battery power (earlier, it skipped unless the command is sure that
we are on ac power).

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