In our backend, we mostly follow the convention that type names
for the resolvers match up with the types in the GraphQL API,
with the extra Resolver suffix. However, this is broken by the
GitTreeEntryResolver where there were no types called
GitBlobResolver or GitTreeResolver, since all functionality
was implemented on top of GitTreeEntryResolver.

Since Go supports implicit method forwarding for embedded structs,
I think we can improve the code consistency & readability by adding
these two stub resolver types.

As struct embedding is not a super common pattern, I've explicitly
mentioned that in the doc comments for GitBlobResolver and
GitTreeResolver.

Test plan

Should be covered by existing tests.

Changelog