Create a shallow clone of the dependent repositories.

No need for the whole history so let's clone only the requested
revision. This speeds up the clone significantly and reduce the required
disk space.

Author: DanielKristofKiss

scripts/repos.py

@@ -263,23 +263,21 @@ def clone_repositories(checkout_path: str, tc_version: LLVMBMTC,
         logging.info(' - %s: %s @ %s%s', repo_path, module.branch,
                      module.revision,
                      ' (detached)' if module.revision != 'HEAD' else '')
+        refspec = (module.branch if module.revision == 'HEAD'
+                   else module.revision)
+
         repo = git.Repo.clone_from(module.url,
-                                   os.path.join(checkout_path, module.name))
-        if module.revision == 'HEAD':
-            try:
-                repo.git.checkout(module.branch)
-            except git.exc.GitCommandError as ex:  # pylint: disable=no-member
-                die('could not checkout "{}" @ "{}/{}"\n'
-                    'Git command failed with:\n{}'
-                    .format(repo_path, module.branch, module.revision, ex))
-        else:
-            # Detached state
-            try:
-                repo.git.checkout(module.revision)
-            except git.exc.GitCommandError as ex:  # pylint: disable=no-member
-                die('could not checkout "{}" @ "{}".\n'
-                    'Git command failed with:\n{}'
-                    .format(repo_path, module.revision, ex))
+                                   os.path.join(checkout_path, module.name),
+                                   multi_options=[
+                                       "--branch %s" % (refspec),
+                                       "--depth 1"
+                                       ])
+        try:
+            repo.git.checkout(module.revision)
+        except git.exc.GitCommandError as ex:  # pylint: disable=no-member
+            die('could not checkout "{}" @ "{}".\n'
+                'Git command failed with:\n{}'
+                .format(repo_path, refspec, ex))
 
     patch_repositories(checkout_path, tc_version, patches)