Stop using shared DeepMerger in ReadContext for executeSelectionSet.
Stop using shared DeepMerger in ReadContext for executeSelectionSet.

Issue #9735 appears to be due to a bug I introduced in my commit
756ab87e01e8d48eab99d82b4806f3fdefb5bf70 in PR #8734.

Premature optimization has a bad reputation already, but you can add
this PR to the mountain of evidence in its disfavor.

apollographql/apollo-client,apollographql/apollo-client