refactor(scm): use scm for mergeBranch, mergeToLocal (#23448)

lib/modules/platform/default-scm.spec.ts

lib/modules/platform/default-scm.ts

lib/modules/platform/local/scm.spec.ts

lib/modules/platform/local/scm.ts

lib/modules/platform/types.ts

lib/util/git/index.spec.ts

lib/util/git/index.ts

lib/workers/repository/onboarding/branch/index.spec.ts

lib/workers/repository/onboarding/branch/index.ts

lib/workers/repository/update/branch/automerge.spec.ts

lib/workers/repository/update/branch/automerge.ts