Automatic merge of local modifications in submodules?

Hello,

I am trying to explore gti's submodule functionality. Since I'm used to
svn:externals, I am missing the automatic merge of local modifications.
So I'd like to do something like:

  (cd submodule; git stash)
  git submodule update
  (cd submodule; git stash pop)

The problem with this command sequence is that it is not atomic. So if it
is automatically executed from a script (e.g cron or post-update-hook), it
will cause damage.

Any ideas how to properly auto-merge local modifications on submodule
update? git-submodule don't seem to have an option to do that.