Output directory is entirely obliterated, including a .git dir

[jshirley]

I am exporting into a gh-pages directory, which has a .git path. Unfortunately, when I export to that directory the entire directory is removed, including .git.

I have this wrapped, but it could be easier to export directly to a git managed directory (specifically in a gh-pages branch).

[davglass]

I'll add a flag to not delete the out directory before writing changes.