Sync FSX with FSFS:
Record-only merge changes from libsvn_fs_fs that either have already
been replaced manually or don't apply to FSX at all.  Revisions:
r1605123, r1636478, r1636483 and r1636629.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1636639 13f79535-47bb-0310-9956-ffa450edef68