One last issue #3020 fix: Correctly map mergeinfo revisions when loading
incremental dumps into a repository that was not empty prior to the first
load.

* subversion/include/private/svn_mergeinfo_private.h

  (svn_mergeinfo__adjust_mergeinfo_rangelists): New.

* subversion/libsvn_subr/mergeinfo.c

  (svn_mergeinfo__adjust_mergeinfo_rangelists): New.

* subversion/libsvn_repos/load.c

  (renumber_mergeinfo_revs): Adjust mergeinfo older than the oldest revision
   in the dump stream by the difference between the head rev of the target
   repository and the current dump stream rev.

  (new_revision_record): Update parse_baton.oldest_old_rev here, a bit sooner
   than we did before when we set it...

  (close_revision): ...here.

* subversion/tests/cmdline/svnadmin_tests.py