package: only download tree & blobs for the commit we want

We can't do a real shallow clone since GitHub doesn't let you fetch
arbitrary commits. Instead we fetch the whole commit graph, but not the
file trees or file blobs, which are fetched by git checkout on-demand.

.github/workflows/build.yml

@@ -29,7 +29,7 @@ jobs:
         path: |
           ~/.gradle/caches/
           ~/.gradle/wrapper/
-        key: package-2.0.6
+        key: package-2.0.7
     - name: prepare
       run: |
         pushd package
@@ -82,7 +82,7 @@ jobs:
         path: |
           ~/.gradle/caches/
           ~/.gradle/wrapper/
-        key: upload-2.0.6
+        key: upload-2.0.7
     - uses: actions/download-artifact@v2
       with:
         name: manifest_diff