Guard against unresolvable root ref in DivergingCommitCountsService

What does this MR do and why?

Fixes a 500 error from Projects::BranchesController#diverging_commit_counts

NoMethodError: undefined method `id' for nil
  app/services/branches/diverging_commit_counts_service.rb:11:in `call'

When a repository's root ref (HEAD) doesn't resolve to a commit, raw_repository.commit(repository.root_ref) returns nil and every diverging counts request 500s. This is reachable via valid git states (unborn HEAD on an empty/renamed default branch, or HEAD pointing at a deleted branch), so a deterministic 500 mischaracterises a representable repository state as a server error.

How to set up and validate locally

1. Point a test repo's HEAD at a non-existent branch (git symbolic-ref HEAD refs/heads/gone on the Gitaly-side repo).
2. Load the Branches page. Before: 500 + "Error fetching diverging counts" alert. After: 200 with {}, page renders cleanly, one UnresolvableRootRefError tracked.

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