Merge branch 'js/scalar-diagnose'

Implementation of "scalar diagnose" subcommand.

* js/scalar-diagnose:
  scalar: teach `diagnose` to gather loose objects information
  scalar: teach `diagnose` to gather packfile info
  scalar diagnose: include disk space information
  scalar: implement `scalar diagnose`
  scalar: validate the optional enlistment argument
  archive --add-virtual-file: allow paths containing colons
  archive: optionally add "virtual" files