Fix occasional destructive re-rooting of a tree.
Fix occasional destructive re-rooting of a tree.

This was due to unexpected reversal (flipping) of a latent (unused)
ad-hoc root edge, which could occur if the ad-hoc rooting elements were
re-purposed for a new rooting operation. Fixes #1035.

OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree,OpenTreeOfLife/opentree