Merge branch 'ab/gc-reflog'

Fix various glitches in "git gc" around reflog handling.

* ab/gc-reflog:
  gc: handle & check gc.reflogExpire config
  reflog tests: assert lack of early exit with expiry="never"
  reflog tests: test for the "points nowhere" warning
  reflog tests: make use of "test_config" idiom
  gc: refactor a "call me once" pattern
  gc: convert to using the_hash_algo
  gc: remove redundant check for gc_auto_threshold