Hello,

Attached is a patch (in git am format) to add a new interactive function
`vc-activate`, which simply calls `vc-find-file-hook` to activate VC
mode on a buffer.

The motivation comes from the fact that I don't consistently use VC for
everything. I often pop back and forth to the command line, depending on
the situation. This ends up with a lot of "git add" commands being run
behind emacs's back. I've been using `M-x revert-buffer` or `C-x C-v`,
but both of them seem too harsh for what I really want, which is for VC
to just check again real quick. I could `M-: RET (vc-find-file-hook)`
but that doesn't roll off the tongue very well.

`vc-activate` is a nicely named interactive wrapper around
vc-find-file-hook so that I can just do `M-x vc-activate RET` and I'm in
business.

Thanks,
  David