Merge branch 'jt/repo-structure'

Merge branch 'jt/repo-structure'

"git repo structure", a new command.

* jt/repo-structure:
  builtin/repo: add progress meter for structure stats
  builtin/repo: add keyvalue and nul format for structure stats
  builtin/repo: add object counts in structure output
  builtin/repo: introduce structure subcommand
  ref-filter: export ref_kind_from_refname()
  ref-filter: allow NULL filter pattern
  builtin/repo: rename repo_info() to cmd_repo_info()