Sync FSX with FSFS:
Cleanly merge r1634875, 1634879 and 1634880 from libsvn_fs_fs.

This ports the latest revprop pack file parsing improvements to FSX.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1634945 13f79535-47bb-0310-9956-ffa450edef68