Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.

I'm not sure if worktree.h was intentionally left out of git2.h. Looks like an oversight since it is in fact documented.

It is currently unused; it will go into the remote's options.

This was forgotten when the feature was implemented.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

Our git_openssl_set_locking() would ideally not exist. Make it clearer
that we provide it as a last resort and you should prefer anything else.

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.