Add reference argument to submodule update --init invocations (#842)

When resolving a repo with submodules via the repo_resolver::checkout() method, the reference argument (if appropriate) was not used for submodule updates as part of resolution. This PR adds support for the reference argument to allow references to be used throughout the resolution process.

edk2toolext/environment/repo_resolver.py

@@ -388,6 +388,9 @@ def checkout(
     """
     details = repo_details(abs_file_system_path)
     with Repo(abs_file_system_path) as repo:
+        reference = None
+        if "ReferencePath" in dep and os.path.exists(dep["ReferencePath"]):
+            reference = Path(dep["ReferencePath"])
         if "Commit" in dep:
             commit = dep["Commit"]
             if update_ok or force:
@@ -397,7 +400,10 @@ def checkout(
                 except GitCommandError:
                     repo.git.fetch()
                     repo.git.checkout(commit)
-                repo.git.submodule("update", "--init", "--recursive")
+                if reference:
+                    repo.git.submodule("update", "--init", "--recursive", "--reference", reference.as_posix())
+                else:
+                    repo.git.submodule("update", "--init", "--recursive")
             else:
                 head = details["Head"]
                 if commit in [head["HexSha"], head["HexShaShort"]]:
@@ -427,7 +433,10 @@ def checkout(
                     repo.remotes.origin.config_writer.set_value("fetch", refspec).release()
                     repo.git.fetch()
                     repo.git.checkout(branch)
-                repo.git.submodule("update", "--init", "--recursive")
+                if reference:
+                    repo.git.submodule("update", "--init", "--recursive", "--reference", reference.as_posix())
+                else:
+                    repo.git.submodule("update", "--init", "--recursive")
             else:
                 if details["Branch"] == dep["Branch"]:
                     logger.debug(