build: actually run import-patches on gclient sync (#17885)

script/lib/git.py

@@ -86,6 +86,17 @@ def apply_patch(repo, patch_path, directory=None, index=False, reverse=False):
   return applied_successfully
 
 
+def import_patches(repo, **kwargs):
+  """same as am(), but we save the upstream HEAD so we can refer to it when we
+  later export patches"""
+  update_ref(
+      repo=repo,
+      ref='refs/patches/upstream-head',
+      newvalue='HEAD'
+  )
+  am(repo=repo, **kwargs)
+
+
 def get_patch(repo, commit_hash):
   args = ['git', '-C', repo, 'diff-tree',
           '-p',