Introduce credential support

[carlosmn]

Implement support for username/password and ssh keypair credentials. Add support for them in the Remote as well as clone_repository(). The parameters expect a callback, but the provided types already provide the right __call__ interface.

There's a bit of sausage-making in this PR, with a bunch of code being removed in the middle, but I've kept it to show the process by which I arrived at this design.

Credentials implement two attributes, and the pygit2 code extracts the information from a tuple and passes it on to libgit2.

[remusavram]

I cloned the latest release: pygit-0.23.3

print help(pygit2.clone_repository)

Help on function clone_repository in module pygit2:

clone_repository(url, path, bare=False, repository=None, remote=None, checkout_branch=None, callbacks=None)
    Clones a new Git repository from *url* in the given *path*.

    Returns a Repository class pointing to the newly cloned repository.

I tried:

pygit2.clone_repository(url, path, credentials=cred)

Get:

    pygit2.clone_repository(url, path, credentials=cred)
TypeError: clone_repository() got an unexpected keyword argument 'credentials'

Question:
How can I clone a remote repository using SSH?

[nick-ulle]

@remusvrm The documentation isn't very clear, but what you need to do is:

callbacks = pygit2.RemoteCallbacks(credentials = cred)
pygit2.clone_repositry(url, path, callbacks = callbacks)