refactor: move branch cache files to util/cache (#18646)

lib/util/cache/branch/behind-base-branch.spec.ts

lib/util/cache/branch/behind-base-branch.ts

lib/util/cache/branch/conflict.spec.ts

lib/util/cache/branch/conflict.ts

lib/util/cache/branch/index.ts

lib/util/cache/branch/modified.spec.ts

lib/util/cache/branch/modified.ts

lib/util/cache/branch/parent-sha.spec.ts

lib/util/cache/branch/parent-sha.ts

lib/util/cache/branch/set-branch-commit.spec.ts

lib/util/cache/branch/set-branch-commit.ts

lib/util/git/index.spec.ts

lib/util/git/index.ts

lib/workers/repository/cache.ts

lib/workers/repository/process/write.ts