Remove a useless common ancestor search from the conflict resolver.
* subversion/libsvn_client/conflicts.c (resolve_incoming_move_dir_merge): Stop searching a YCA and then using it as the left-side of the merge source when merging into the move target. Doing so is wrong because it could lead to dubious conflicts since we will end up ignoring mergeinfo. Use the 'incoming old' path and revision as the left-merge side instead, which is properly bound by merge-tracking done during the merge which recorded the conflict. Problem found by code inspection while working on r1851913.