Merge branch 'lo/repo-struct-z'

"git repo struct" learned to take "-z" as a synonym to "--format=nul".

* lo/repo-struct-z:
  repo: add -z as an alias for --format=nul to git-repo-structure
  repo: use [--format=... | -z] instead of [-z] in git-repo-info synopsis
  repo: remove blank line from Documentation/git-repo.adoc