utils/update-checkout: Handle missing repository/tag combinations.

Fixes https://bugs.swift.org/browse/SR-3810

Author: Doug Coleman

utils/update-checkout

@@ -91,8 +91,10 @@ def update_repository_to_tag(args, repo_name, repo_path, tag_name):
         tag_exists = shell.capture(['git', 'ls-remote', '--tags',
                                     'origin', tag_name], echo=False)
         if not tag_exists:
-            print("Skipping tagging of '" + repo_name + "', tag does not exist")
-            return
+            print("Tag '" + tag_name + "' does not exist for '"
+                    + repo_name + "', just updating regularly")
+            tag_name = None
+
         return [repo_path,
                 tag_name,
                 args.reset_to_remote,