Sync'ing FSX with FSFS:

Port 'svnadmin pack' improvements from FSFS to FSX and record revisions
1555109,1555138,1555250,1555293,1555441,1559886,1570679,1572512,1572514,
1575108,1575429 as being merged from /subversion/libsvn_fs_fs into
subversion/libsvn_fs_x.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1596290 13f79535-47bb-0310-9956-ffa450edef68