Fixes #4438

Plumbing

• Add branchComparisons field to IDailyMeasures in stats database
• Add comparisonsToMaster defaultBranchComparisons field to IDailyMeasures in stats database
• Add mergesInitiatedFromComparison field to IDailyMeasures in stats database
• Add updateFromDefaultBranchMenuCount field to IDailyMeasures in stats database
• Add mergeIntoCurrentBranchMenuCount field to IDailyMeasures in stats database
• Add methods for each new measure to statsStore
• Add a new fields for the new measures to DefaultDailyMeasure in statsStore

Back-end (Central)

• Add measure to desktop_usage_event.rb
• Update usage_test.rb
• Update fake_desktop_data in helper.rb

Database

• Open an issue on github/analytics to get new field pulled into view, update airflow, and update reports

Other

• Update the usage-data page on desktop.github.com

This depends on PRs in central, desktop.github.com, and analytics being merged.