get, drop: Added --auto option, which decides whether to get/drop content
as needed to work toward the configured numcopies.

The problem with bundling it up in optimize was that I then found I wanted
to run an optmize that did not drop files, only got them. Considered adding
a --only-get switch to it, but that seemed wrong. Instead, let's make
existing subcommands optionally smarter.

Note that the only actual difference between drop and drop --auto is that
the latter does not even try to drop a file if it knows of not enough
copies, and does not print any error messages about files it was unable to
drop.

It might be nice to make get avoid asking git for attributes when not in
auto mode. For now it always asks for attributes.