Fix marking static delta commits as partial

This patch makes it so that we mark the .commit file from a static delta
as partial before writing the commit to the staging directory. This
exactly mirrors what we do in meta_fetch_on_complete() when writing the
commit on that codepath, which should lend some credibility to the
correctness of this patch.

I have checked that this fixes an issue Flatpak users have been
encountering (https://github.com/flatpak/flatpak/issues/3479) which
results in error messages like "error: Failed to install
org.freedesktop.Sdk.Extension.texlive: Failed to read commit
c7958d966cfa8b80a42877d1d6124831d7807f93c89461a2a586956aa28d438a: No
such metadata object
8bdaa943b957f3cf14d19301c59c7eec076e57389e0fbb3ef5d30082e47a178f.dirtree"

Here's the sequence of events that lead to the error:
1. An install operation is started that fetches static deltas.
2. The fetch is interrupted for some reason such as network connectivity
   dropping.
3. The .commit and .commitmeta files for the commit being pulled are
   left in the staging dir, e.g.
   "~/.local/share/flatpak/repo/tmp/staging-dfe862b2-13fc-49a2-ac92-5a59cc0d8e18-RURckd"
4. There is no `.commitpartial` file for the commit in
   "~/.local/share/flatpak/repo/state/"
5. The next time the user attempts the install, libostree reuses the
   existing staging dir, pulls the commit and commitmeta objects into
   the repo from the staging dir on the assumption that it's a complete
   commit.
6. Flatpak then tries to deploy the commit but fails in
   ostree_repo_read_commit() in flatpak_dir_deploy(), leading to the
   error message "Failed to read commit ..."
7. This happens again any subsequent time the user attempts the install,
   until the incomplete commit is removed with "flatpak repair --user".

I will try to also add a workaround in Flatpak so this is fixed even
when Flatpak links against affected versions of libostree.

src/libostree/ostree-repo-pull.c

@@ -2244,6 +2244,9 @@ process_one_static_delta (OtPullData                 *pull_data,
                                          ref, cancellable, error))
             return FALSE;
 
+          if (!ostree_repo_mark_commit_partial (pull_data->repo, to_revision, TRUE, error))
+            return FALSE;
+
           if (detached_data && !ostree_repo_write_commit_detached_metadata (pull_data->repo,
                                                                             to_revision,
                                                                             detached_data,