chore(vcs): refactor getCommits to use errgroup

[marcsanmi]

Follow-up PR for #3529 (comment).

Refactor the getCommits function in the VCS service to use errgroup instead of sync.WaitGroup.

Changes

• Implement errgroup for better concurrency management
• Set a limit (10) on concurrent requests to GitHub API

Note on error handling

I kept the existing error handling approach, collecting errors explicitly through the channel rather than relying on errgroup's built-in error handling, since it will only return the first encountered error.

[bryanhuhta]

This looks good, I forgot that errgroup only returns the first error. That's pretty lame. Anyway, at least we have some concurrency limits here so we don't gobble up our entire ratelimit budget in one RPC 😁