unpack-supportconfig: make work with any archive name

Previously, with an archive called something like

  bug-872798_nts_d00-1e-67-2a-ba-68_140409_1618.tbz

if it contained a single top-level directory like

  nts_d00-1e-67-2a-ba-68_140409_1618

which didn't match the basename of the archive, then
unpack-supportconfig would fail.