This option does not get persisted to disk, which makes it different
from the rest of the setters. Remove it until we go all the way.

We still respect the configuration option, and it's still possible to
perform a one-time prune by calling the function.

For each remote-tracking branch we want to remove, we need to consider
it against every other refspec in case we have overlapping refspecs,
such as with

    refs/heads/*:refs/remotes/origin/*
    refs/pull/*/head:refs/remotes/origin/pr/*

as we'd otherwise remove too many refspecs.

Create a list of condidates, which are the references matching the rhs
of any active refspec and then filter that list by removing those
entries for which we find a remove reference with any active
refspec. Those which are left after this are removed.

This makes a fetch+prune more similar to a connect+prune and makes it
more likely that we see errors in the decision to prune a reference.

This is hiding a bug in the prune code, whereby we prune references we
shouldn't but don't notice it in the code afterwards because
update_tips() recreates them.

This means that we do perform changes to the references (and get rid of
the reflogs) when we shouldn't.

We load the remote again, so we need to ask the new remote to prune the
refs, or we're not exercising the code in our tests.

Fold `git_push_unpack_ok()` into `git_push_finish()`

This should make it clearer what the return value implies.

The push cannot be successful if we sent a bad packfile. We should
return an error in that case instead of storing it elsewhere.

Introduce stackable IO streams

Minor Readme improvments

Most of the network-facing facilities have been copied to the socket and
openssl streams. No code now uses these functions directly anymore, so
we can now remove them.

Having an ssh stream would require extra work for stream capabilities we
don't need anywhere else (oob auth and command execution) so for now
let's move away from the gitno connection to use socket_stream.

We can introduce an ssh stream interface if and as we need it.

Win32 leaks

This unfortunately isn't as stackable as could be possible, as it
hard-codes the socket stream. This is because the method of using a
custom openssl BIO is not clear, and we do not need this for now. We can
still bring this in if and as we need it.

We currently have gitno for talking over TCP, but this needs to know
about both plaintext and OpenSSL connections and the code has gotten
somewhat messy with ifdefs determining which version of the function
should be called.

In order to clean this up and abstract away the details of sending over
the different types of streams, we can instead use an interface and
stack stream implementations.

We may not be able to use the stackability with all streams, but we
are definitely be able to use the abstraction which is currently spread
between different bits of gitno.

Update clar to e3985dd

Fix broken tests on Windows