Merge branch 'lo/repo-info-keys' into seen

"git repo info" learns "--keys" action to list known keys.

* lo/repo-info-keys:
  repo: add new flag --keys to git-repo-info
  repo: add a default output format to enum output_format

diff --cc Documentation/git-repo.adoc
Simple merge

diff --cc builtin/repo.c
Simple merge

diff --cc t/t1901-repo-structure.sh
Simple merge