Merge branch 'ua/some-builtins-wo-the-repository'

Merge branch 'ua/some-builtins-wo-the-repository'

A handful of built-in command implementations have been rewritten
to use the repository instance supplied by git.c:run_builtin(), its
caller.

* ua/some-builtins-wo-the-repository:
  builtin/checkout-index: stop using `the_repository`
  builtin/for-each-ref: stop using `the_repository`
  builtin/ls-files: stop using `the_repository`
  builtin/pack-refs: stop using `the_repository`
  builtin/send-pack: stop using `the_repository`
  builtin/verify-commit: stop using `the_repository`
  builtin/verify-tag: stop using `the_repository`
  config: teach repo_config to allow `repo` to be NULL