Optimize `head_tree` code

What does this MR do and why?

Contributes to #386627 (closed)

Problem

Each request to fetch readme_path or avatar_path triggers two Gitaly requests: FindCommit and GetTreeEntries. It's not efficient, because FindCommit is not necessary.

Solution

Remove FindCommit request and rely on the default branch to fetch repository tree.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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