Fix pushing of manifests whose layers share blobs

If a blob was repeated previously, we would get a constraint error from the ManifestBlob table

test/registry/registry_tests.py

@@ -1104,3 +1104,25 @@ def test_login_scopes(username, password, scopes, expected_access, expect_succes
                                  {'SERVER_HOSTNAME': 'localhost:5000'})
   assert payload is not None
   assert payload['access'] == expected_access
+
+
+def test_push_pull_same_blobs(pusher, puller, liveserver_session, app_reloader):
+  """ Test: Push and pull of an image to a new repository where a blob is shared between layers. """
+  credentials = ('devtable', 'password')
+
+  layer_bytes = layer_bytes_for_contents('some contents')
+  images = [
+    Image(id='parentid', bytes=layer_bytes, parent_id=None),
+    Image(id='someid', bytes=layer_bytes, parent_id='parentid'),
+  ]
+
+  options = ProtocolOptions()
+  options.skip_head_checks = True # Since the blob will already exist.
+
+  # Push a new repository.
+  pusher.push(liveserver_session, 'devtable', 'newrepo', 'latest', images,
+              credentials=credentials, options=options)
+
+  # Pull the repository to verify.
+  puller.pull(liveserver_session, 'devtable', 'newrepo', 'latest', images,
+              credentials=credentials, options=options)