Imported Debian patch 1.2.8-5ubuntu3
Add --no-fsync parameter.

If this parameter is given, rdiff-backup will skip all calls to
fsync(2).

This can give dramatic performance improvements on some workloads,
especially workloads involving lots of small changes. However, there
is the possibility that after an unexpected power-loss the backup
repository is broken.
Add test to measure the cost of fsync().
Merge upstream version 1.2.8-7 (Ubuntu 14.04)