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