vc-register complains if a file is already registered in a git repository

[Phil Hagelberg <phil@hagelb.org>]

When I visit a file that is already registered in my git repository, I
would like to make some changes and register them in the git index.

Unfortunately when I use vc-register on a file that is already in the
repository, it complains, saying "This file is already registered". This
is a reasonable thing to do in most VC systems, but git behaves
differently since it's a common thing to want to register the changes
you've just made to the git index (or staging area).

I would suggest that the vc-register be allowed when the file is
already registered if the backend is git. I know that VC intends to be a
fairly transparent frontend that behaves the same way with all different
version control systems, but ignoring the existence of the git index
discards a lot of the flexibility that is the reason many people use git
in the first place.

Maybe if a special-case within vc.el itself is frowned upon a new
command such as vc-register-changes could be made that is only
implemented in the git backend?

-Phil