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