Backport change from trunk:
backends/chert/chert_btreebase.cc,backends/chert/chert_table.cc:
Only full sync the final base file written.

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18276 6b2f1b92-a4f3-0310-943f-de3cbe41748b