git: Automatically reapply stashed changes if possible.
In the event that the stash does not apply a "post-build" message is issued to warn the user. Otherwise there is an in-situ message. This addresses user feedback I have received on Invent and IRC. In particular re-fixes #42 and bug 420933, and fixes #49. All that I do here is to use the existing logic and add "git stash pop" after the update phase has completed. I've previously tried --autostash (see !32 and !45) and that had issues of its own. Using this approach allows us to use rely on git directly without risk of missing a merge conflict that git sometimes seems to generate with `git pull --autostash` (see !32 (comment 46926)). The merge request !61 may still be applicable. I haven't looked long enough to tell if there's more on it than the --autostash feature which still causes us problems... it's possible that there's a more reliable method than what we're doing though we've already taken pains to simplify this routine because of the recurring stash failures that we've seen previously. BUG:420933