lib: Use OtTmpFile for static delta processing

The `OstreeRepoContentBareCommit` struct was basically an `OtTmpFile`, so let's
make it one. I moved the "convert to `GOutputStream`" logic into the callers,
since that bit can't fail; it makes the implementation much simpler since we can
just return the result of `ot_open_tmpfile_linkable_at()`.

Prep for `GLnxTmpfile` porting.

Closes: #957
Approved by: jlebon

src/libostree/ostree-repo-pull.c

@@ -1108,7 +1108,7 @@ meta_fetch_on_complete (GObject           *object,
       goto out;
     }
 
-  /* Now delete it, keeping the fd open as the last reference); see comment in
+  /* Now delete it, keeping the fd open as the last reference; see comment in
    * corresponding content fetch path.
    */
   ot_cleanup_unlinkat (&tmp_unlinker);