UI improvements for git branches

[Dan Nicolaescu]

Currently C-u C-x v s can be used to create Git branches. 
C-x v r can be used to switch to a different branch.
All buffers are updated correctly after these operations.

But this functionality is rather obscure.

The function names are not very intuitive and easy to find.

There's no completion (C-x v r would be easier to use if it had branch
name completion).

How can we improve Git branch support?