vc-magit mode (was: Re: PROPOSAL: Repurpose one key and reserve it for third-party packages)

[Alfred M. Szmidt]

Here is an idea, a new backend -- vc-magit.  This calls the right
Magit functions.  The user can then set the prefered backends using
vc-handled-backends.  This solves the (specific) conundrum of taking
e.g., C-x g, and also provides the user with a consitent interface to
VC mode when using magit.

When in a buffer that has a git repository associated and your backend
is vc-magit, vc-prefix-map, can be used by vc-magit / magit to bind
specific magit keybindings that are not VC agonstic.