Reduce the number of Gitaly calls for submodules

Igor Drozdovrequested to merge
id-submodule-url-graphql into master

What does this MR do?

Optimize the number of Gitaly calls for submodules. This commit optimizes the backend performance of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29261

Related issue

https://gitlab.com/gitlab-org/gitlab-ce/issues/61578

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