Make a backport branch for r1574868.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1574868@1575329 13f79535-47bb-0310-9956-ffa450edef68
On the 1.8.x-r1574868 branch: merge r1574868 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1574868@1575330 13f79535-47bb-0310-9956-ffa450edef68
Merge the 1.8.x-r1574868 branch:

 * r1574868
   Don't let invalid mergeinfo stop 'svnadmin dump' from producing a dump.
     Part of issue #4476 "Mergeinfo containing r0 makes svnsync and svnadmin
     dump fail".
   Justification:
     A formatting error in what is essentially client-side metadata, albeit
     with repository-side interpretation as well, should never prevent
     dumping the repository. This issue only occurs when svnadmin is trying
     to warn us of mergeinfo revisions that might be unexpected, so it is
     not essential.
   Notes:
     There is a work-around, at least in theory: a dump starting from
     revision 0 or 1 does not suffer from this problem.
   Branch:
     ^/subversion/branches/1.8.x-r1574868
   Votes:
     +1: julianfoad, philip, rhuijben


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1579874 13f79535-47bb-0310-9956-ffa450edef68