Avoid Gitaly RPC errors when fetching diff stats

What does this MR do?

Instead making the roundtrip to Gitaly in order to not find the BLANK_SHA on DiffStats RPC and raise a rpc error: code = Unavailable desc = DiffStats: exit status 128, this MR treats this scenario in the client side.

We should stop seeing https://sentry.gitlab.net/gitlab/gitaly-production/issues/531028/ after getting this in.

What are the relevant issue numbers?

Fixes gitaly#1393 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.