Merge branch 'dt/gc-ignore-old-gc-logs'

A "gc.log" file left by a backgrounded "gc --auto" disables further
automatic gc; it has been taught to run at least once a day (by
default) by ignoring a stale "gc.log" file that is too old.

* dt/gc-ignore-old-gc-logs:
gc: ignore old gc.log files

diff --cc Documentation/config.txt
Simple merge