[update-checkout] When branch is not set, use the default-branch-alias specified by our config file.

Certain checkout configurations do not have the proper default branches set. We
delegate to the config file to specify the default branches to use so the config
file can just tell us the appropriate thing to do here.

Author: gottesmm

utils/update-checkout

@@ -153,6 +153,11 @@ By default, updates your checkouts of Swift, SourceKit, LLDB, and SwiftPM.""")
         config = json.load(f)
     validate_config(config)
 
+    # If branch is None, default to using the default branch alias specified by
+    # our configuration file.
+    if branch is None:
+        branch = config['default-branch-alias']
+
     if clone or clone_with_ssh:
         obtain_additional_swift_sources(
             config, clone_with_ssh, branch, skip_history, args.skip_repository)

utils/update-checkout-config.json

@@ -16,6 +16,7 @@
         "swift-integration-tests": "apple/swift-integration-tests"
     },
     "branch_names": ["master-branches", "next-branches", "swift-3.0-preview-1-branches"],
+    "default-branch-alias": "master",
     "master-branches": {
         "aliases": ["master", "stable"],
         "repos": {