Merge branch 'es/mark-gc-cruft-as-experimental'

Enable gc.cruftpacks by default for those who opt into
feature.experimental setting.

* es/mark-gc-cruft-as-experimental:
  config: let feature.experimental imply gc.cruftPacks=true
  gc: add tests for --cruft and friends

diff --cc builtin/gc.c
Simple merge

diff --cc repo-settings.c
Simple merge

diff --cc repository.h
Simple merge