Gitaly: Add batch version of CommitStats RPC

Problem

I'm working on a service that sends branch data to Jira (gitlab#263240 (closed)). Currently, we sync branch data only on events, like when a new branch is created. But we also want to be able to sync multiple branches in one go when a new Project gets connected to Jira.

The data includes the number of files that were changed. We currently use the CommitStats RPC but we ran into an N+1 problem because it is not possible to batch load stats for multiple commits. I tried it in this MR: gitlab!48706 (comment 459086632).

Possible solutions

Create a batch version of the CommitStats RPC

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