Add temporary dir/file for htools import

In order for “git merge -s recursive -X subtree=htools” to do its
magic, an (mostly empty) directory must exist in the target tree name
‘htools’. Hence we add a temp file to it before the merge, and remove
it after that.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>