lib/commit: Use direct repo writes if fsync is disabled

lib/commit: Use direct repo writes if fsync is disabled

For situations where fsync is disabled, there's basically
no reason to do the whole "staging directory" dance.  Just
write directly into the repo.

Today I use `fsync=false` for my build/cache repos.

I briefly considered not allocating a tmpdir at all
in this case, but we actually do want the txn tmpdir
for the non-`O_TMPFILE` case.

Part of https://github.com/ostreedev/ostree/issues/1184

Closes: #1354
Approved by: giuseppe