How to pre-empt git pull merge error?

I am looking for a way of detecting up front whether a git pull or git merge would fail. The sort of script I want to perform is to update a server.

    git fetch
    git okay
    stop server
    backup data
    git merge
    start server

Here git okay is a place holder for the command I am asking for.

If a file has been changed outside of a commit then git pull fails with the following error.

error: Your local changes to '...' would be overwritten by merge. Aborting. Please, commit your changes or stash them before you can merge.

I would like git okay to perform the pre-merge checks described in git merge and return non-zero status so that the script aborts before the server is stopped.

Possibilities I have looked for and not found include git merge --dry-run. My best line of thought is git status --porcelain | pre-merge-okay. That seems like a lot of work to make a pre-merge-okay that deals with things like benign untracked files.

I have also asked this question on Stack Overflow but received no answers.

http://stackoverflow.com/questions/20221383/