When git --aggressive is run on a bare repo, it deletes all head refs. This change copies the contents into memory and then writes them out at the end of the repo:gc call.
Closes#6973
This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.
This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.
This reduces duplication and allows users to rely on global arguments to generate most labels needed.
The 'commit' command cannot have global arguments because the whitespace within the label change breaks string-based interpolation, and shell arrays cannot be exported.
