[update-checkout] Only use the default-branch-scheme when cloning.

We want to allow for people to use update-checkout to just update branches even
in the case where they are not using a scheme (for instance if they have a side
branch).

This means though that one can still clone and get all of the appropriate
repos/branches.

The only unfortunate thing about this change is that when cloning, already
cloned repositories that already are setup to a branch will have their branch
changed.

It would be good at some point to separate updating from cloning. It would make
update-checkout much cleaner.

Author: gottesmm

utils/update-checkout

@@ -196,12 +196,12 @@ 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 scheme is None:
-        scheme = config['default-branch-scheme']
-
     if clone or clone_with_ssh:
+        # If branch is None, default to using the default branch alias specified by
+        # our configuration file.
+        if scheme is None:
+            scheme = config['default-branch-scheme']
+
         obtain_additional_swift_sources(
             config, clone_with_ssh, scheme, skip_history,
             args.skip_repository_list)