I spent a long time worrying about this problem with --all, that it cannot
check .gitattributes files for numcopies settings, and so would not be
entirely safe to use. The solution turns out to be simple, just don't
implement `git annex drop --all`. drop is the only command that needs to
check numcopies (move can also reduce the number of copies, but explicitly
bypasses numcopies settings).
Use cases that might need a drop --all are probably better served by using
unused and dropunused, which already work in a bare repository.
ebfd6fc2fe
