[clang-diff] Fix some errors and inconsistencies

Fix to the computation of the rightmost descendant.

Prevents root nodes from being mapped if they are already mapped.  This
only makes a difference when we compare AST Nodes other than entire
translation units, a feature which still has to be tested.

Reviewers: arphaman

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D36176

llvm-svn: 311172