Sync FSX with FSFS:
Manually port revisions r1605444, r1606554 and r160674 from libsvn_fs_fs.

These patches switch writing revision zero from using a full template to
a generating the index sections.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635598 13f79535-47bb-0310-9956-ffa450edef68