libgit2 implementations of smart subtransports can simply reach through
the structure, but external implementors cannot.

Add these two functions as a way for the smart subtransports to get the
callbacks as set by the user.

When a configuration file is locked, any updates made to it will be done
to the in-memory copy of the file. This allows for multiple updates to
happen while we hold the lock, preventing races during complex
config-file manipulation.

Resolve documentation warnings

Removing a reflog upon ref deletion is something which only some
backends might wish to do. Backends which are database-backed may wish
to archive a reflog, log-based ones may not need to do anything.

Allow custom filters with wildcard attributes, so that clients
can support some random `filter=foo` in a .gitattributes and look
up the corresponding smudge/clean commands in the configuration file.

If the stream claims to support this feature, we can let the transport
set the proxy.

We also set HTTPPROXYTUNNEL option so curl can create a tunnel through
the proxy which lets us create our own TLS session (if needed).