Bug fix in heap unionfind (ticket #197)

The minimum item in the unionfind tree might become inconsistent when
the split operation merges two subtrees which have equal keys. The
current changeset fix the problem. It also fix a wrong index.

if WANT_TOOLS

bin_PROGRAMS +=

dist_bin_SCRIPTS += tools/lemon-0.x-to-1.x.sh

endif WANT_TOOLS