Perform the advice only if the file is into a git repository

When the file made read only is not in a git repository, doing anything for git annex makes no sense.

Moreover, the call to git annex add makes emacs hang for some seconds. This is really annoying.

Thus, I propose to avoid doing anything when not in a git repository by encapsulating the code by the following:

 (when (string=
     (vc-backend buffer-file-name)
      "Git"
      )
...
)