Consider making `nano-modeline-git-info` VC-backend-agnostic

[librarianmage]

Right now, the nano-modeline-git-info function tries to truncate Git- from vc-mode. However, this interacts oddly with other VC backends, which are not necessarily three letters long

More info:

• https://www.gnu.org/software/emacs/manual/html_node/emacs/VC-Mode-Line.html
• https://emacs.stackexchange.com/questions/61837/how-to-query-the-name-of-the-current-branch-in-an-elisp-function

[rougier]

Hence the name nano-modeline-git-info and not nano-modeline-vc-info... Just kidding. Do you want to make a PR? Else, I can try to do it but not before a few days.