magit.el: add `vc-follow-symlinks' to 'magit customization group

Make users aware of `vc-follow-symlinks's existence, and the fact
that it's relevant to Magit.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge praet/vc-follow-symlinks
magit-key-mode.el: make faces customizable

As requested [1] by  Phil Jackson <phil@shellarchive.co.uk>.

[1] mid:"CAFf4W1ZXpwRtPLCMULSm8h50fJHUvGzUtOw32xrzhMVgG870Zg@mail.gmail.com"

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge branch 'praet/key-mode-faces'