Fix a bug that causes repeat merges (and conflicts) when the merge target
has subtrees with explicit mergeinfo and those subtrees need ranges merged
that are older than, and don't intersect with, the oldest ranges needed
by the target.

* subversion/libsvn_client/merge.c
  (drive_merge_report_editor): When describing the merge target in the
  initial reporter set_path call, be sure to describe a revision that falls
  within the range of revisions being merged during this editor drive.