clone: add a callback for repository creation

Analogously to the remote creation callback, provide a way for the user
of git_clone() to create the repository with whichever options they
desire via callback.
clone: remote git_clone_into{,_local} from the public API

As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.
Merge pull request #2452 from libgit2/cmn/clone-custom-repo

Provide a callback to customize the repository on clone