Allow ignoring untracked files when ad-hoc packaging with Git

[akx]

Since we use git ls-files -ocz, untracked files also get included in ad-hoc runs, which may not always be what you want.

This behavior should be configurable, likely by way of an top-level command line parameter (which can also be an envvar thanks to Click).

The option should drop the o, so make it git ls-files -cz.