Merge branch 'gc/bare-repo-discovery'

Introduce a discovery.barerepository configuration variable that
allows users to forbid discovery of bare repositories.

* gc/bare-repo-discovery:
  setup.c: create `safe.bareRepository`
  safe.directory: use git_protected_config()
  config: learn `git_protected_config()`
  Documentation: define protected configuration
  Documentation/git-config.txt: add SCOPES section