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 ostreedev#1184

Closes: ostreedev#1354
Approved by: giuseppe

src/libostree/ostree-repo-commit.c

@@ -38,13 +38,14 @@
 #include "ostree-checksum-input-stream.h"
 #include "ostree-varint.h"
 
-/* In most cases, we write into a staging dir for commit, but we also allow
- * direct writes into objects/ for e.g. hardlink imports.
+/* If fsync is enabled and we're in a txn, we write into a staging dir for
+ * commit, but we also allow direct writes into objects/ for e.g. hardlink
+ * imports.
  */
 static int
 commit_dest_dfd (OstreeRepo *self)
 {
-  if (self->in_transaction)
+  if (self->in_transaction && !self->disable_fsync)
     return self->commit_stagedir.fd;
   else
     return self->objects_dir_fd;