Enable multi-process commit mining in HTTP service

Since bugbug is no longer hosted on Heroku, memory constraints that required single-process commit mining are no longer relevant. This restores parallel processing for performance.

Changes

• Remove use_single_process=True from repository.download_commits() calls in:
  • http_service/bugbug_http/boot.py - commit DB updates during worker boot
  • http_service/bugbug_http/models.py - patch analysis for test selection

The function defaults to use_single_process=False, which enables ProcessPoolExecutor for parallel commit and patch processing across all available CPU cores.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Use multiple processes for mining commits in the HTTP service</issue_title> <issue_description>Since we will no longer host bugbug on Heroku, we are not as concerned with memory usage as before.

Basically reverting b74bf96400d0e845705ae41ec308119125fa31ab and 2ac0bc4c12bf0bd37abcbd631ef4f503a46aeee5 will do the trick.</issue_description>

<agent_instructions>Follow the comments and issues mentioned. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes mozilla/bugbug#5387

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Let's wait a bit on this until the new deployment is fully functional.