Refactor the UpdateMirrorService

What does this MR do and why?

Related to: #561037

This is a follow up to LFS mirroring timeout issue (!198736 - merged)

This MR extracts branch filtering and change calculation logic into separate service classes for better maintainability and performance.

• Add BranchFilter service to handle protected branch and regex filtering.
• Add BranchChangesCalculator service to determine which revisions changed

References

How to set up and validate locally

1. Create a remote project (or you can use the exiting one https://gitlab.com/emmaspark/mirroring-lfs)
2. Create an empty local project WITHOUT a README file
3. Go to Settings → Repository → Mirroring
4. Add the remote project repository URL (e.g., https://gitlab.com/emmaspark/mirroring-lfs.git)
5. Confirm it's pull mirroring
6. Click "Mirror repository"
7. Check if the initial mirroring is successful
8. A new branch + the existing branches test:

• In the remote repo: add a new branch with a new LFS file(file_name.txt). Also edit one of the existing branches.
• In the local repo: go to Settings → Repository → Mirroring. Click the run icon next to a bin icon. Check your local repo and see if both branches are mirrored.

8. The protected branch test:

• In the local repo: repeat the step 2-5 and check "Mirror only protected branches" -> the step 7 and 8 -> set the protected branch in Settings -> Protected branches
• In the remote repo: edit one of the existing branches(an unprotected branch) and the protected branch as well. Start mirroring. See if the only protected branch is mirrored.

MR acceptance checklist

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Related to #561037