Backport r1475724 to 1.7.x.

Attempting to record the mergeinfo on the branch root raised a tree conflict
due to svn.c not existing, and would have caused N mergeinfo changes anyway
on random files in the branch, so I skipped that.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.7.x-r1475724@1475745 13f79535-47bb-0310-9956-ffa450edef68