git-new-workdir: don't fail if the target directory is empty
git-new-workdir: don't fail if the target directory is empty

Allow new workdirs to be created in an empty directory (similar to "git
clone").  Provide more error checking and clean up on failure.

Signed-off-by: Paul Smith <a027184a55211cd23e3f3094f1fdc728df5e0500@mad-scientist.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git