fix for edge case where leaf is mapped to the mrca taxon of the whole tree
Merge branch 'master' of github.com:rhr/ivy

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

* examples/treegraph-test/fetch-strees.py: wip