Sync FSX with FSFS.  Block small patches to code that does not exist in FSX:
1637363, r1640062 and r1640966.

* subversion/libsvn_fs_x
  (): Update mergeinfo.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1645590 13f79535-47bb-0310-9956-ffa450edef68