WIP: Use Git to create a temp stash and an archive from it to create a separate source tree

The problem here is that `git stash create` has no --include-untracked
option, so only modified files are included not new ones that have not
yet been added to the index.

It is also quite a bit slower for incremental builds as Git insists on
setting the modification time to the current time for every file. Which
probably makes sense as Git does not keep track of timestamps and Git
archive does not operate on the work tree.