libostree: Add _finish() API to async progress

Since OstreeAsyncProgress queues to the mainloop, we might "lose" the
last message.  Give callers a way to force a flush.

src/libostree/ostree-sysroot-upgrader.c

@@ -452,6 +452,9 @@ ostree_sysroot_upgrader_pull (OstreeSysrootUpgrader  *self,
                              flags, progress,
                              cancellable, error))
         goto out;
+
+      if (progress)
+        ostree_async_progress_finish (progress);
     }
 
   if (!ostree_repo_resolve_rev (repo, origin_refspec, FALSE, &self->new_revision,