Merge pull request #3117 from apple/fix-update-checkout

[utils] Fix submodule logic for update-checkout script

Author: shahmishal

utils/update-checkout

@@ -122,7 +122,10 @@ def obtain_additional_swift_sources(
                 shell.call(['git', '--git-dir', src_path, '--work-tree',
                             os.path.join(SWIFT_SOURCE_ROOT, repo_name),
                             'checkout', repo_branch], echo=False)
-
+    with shell.pushd(os.path.join(SWIFT_SOURCE_ROOT, repo_name),
+                         dry_run=False, echo=False):
+        shell.call(["git", "submodule", "update", "--recursive"],
+                             echo=False)
 
 def validate_config(config):
     # Make sure that our branch-names are unique.