Cope when an orig tarball is a tarbomb.

Cope when an orig tarball is a tarbomb.

Ie, if it contains other than one single directory toplevel.
Closes:#843422.

Specifically:

 * We change the unpack directory to _unpack-tar in unpack, rather
   than ../unpack-tar.  This means that mktree_in_ud DTRT in that
   directory, rather than in its supposed only subdiredctory
 * We do write-tree on the whole tarball.
 * Then if the tarball had only one subdir in it, we fish the
   subtree's tree object out of the git tree object.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>