lib/repo: Don't syncfs or fsync() dirs if fsync opt is disabled

lib/repo: Don't syncfs or fsync() dirs if fsync opt is disabled

There are use cases for not syncing at all; think build cache repos, etc. Let's
be consistent here and make sure if fsync is disabled we do no sync at all.

I chose this opportunity to add tests using the shiny new strace fault
injection.  I can forsee using this for a lot more things, so I made
the support for detecting things generic.

Related: https://github.com/ostreedev/ostree/issues/1184

Closes: #1186
Approved by: jlebon