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

Fix negative ignores withing ignored dirs

doc: add documentation to all the public structs and enums

Adjust the local transport for the common refspec parser

Notes API consistency fixes

The iterator is the last argument. There is also no returned notes, just
ids, so the comment about freeing is out of place.

Make it consistent between git_note_create() and git_note_remote() by
putting it after the repository.

This makes them show up in the reference, even if the text itself isn't
the most descriptive.

These have been found with

    grep -Przon '\n\ntypedef struct.*?\{' -- include
    grep -Przon '\n\ntypedef enum.*?\{' -- include

A rule can only negate something which was explicitly mentioned in the
rules before it. Change our parsing to ignore a negative rule which does
not negate something mentioned in the rules above it.

While here, fix a wrong allocator usage. The memory for the match string
comes from pool allocator. We must not free it with the general
allocator. We can instead simply forget the string and it will be
cleaned up.

Given

    top
    !top/foo

in an ignore file, we should not unignore top/foo. This is an
implementation detail of the git code leaking, but that's the behaviour
we should show.

A negation rule can only negate an exact rule it has seen before.