Implement git-handler.el, a file-handler for blobs and trees

[tarsius]

This description was taken from #2956. I intend to replace it with a more in-depth description at a later time.

A number of Git related packages, including Magit, allow visiting blobs in a buffer. These buffers are a lot like file-visiting buffers, except they are not. The result is that the buffers created by one of these packages is only barely useful for that package that created them and not compatible with any of the other packages.

This can be fixed by turning blob-visiting buffers into actual file-visiting buffers. That doesn't mean that a file has to be created, it only means that a file-handler has to be defined, a feature provided by Emacs which was originally implemented to allow accessing remote files as if they were local files. In the case of blobs, a file-handler would allow accessing past versions of a file as if it was the current version.

This file-handler will likely be called just git-handler.el.