Direct mode repositories can now have core.bare=true set, to prevent
accidentally running git commands that try to operate on the work tree,
and so do the wrong thing.
This is not yet the default, and it causes known problems for git-annex sync
due to receive.denyCurrentBranch not working in bare repositories.
This commit was sponsored by Richard Hartmann.
This was the last place in git-annex that could remove data referred to by
the git history, without being forced.
Like drop, dropunused checks remotes, and honors the global annex.numcopies
setting. (However, .gitattributes settings cannot apply to unused files.)
