Do not use unknown variable

Probably leftover from slightly different implementation.  There's
nothing to add to the error message now.

src/include/release.sh

         not_dirty)
             git diff --shortstat 2>/dev/null \
               | grep -q . \
-             && die "tree is dirty: $dirt"
+             && die "tree is dirty!"
             ;;
         tags_ok)
             git_fact latest_tag \