build: use --keep-non-patch flag with git am (#48797)

script/lib/git.py

@@ -52,7 +52,8 @@ def get_repo_root(path):
 
 def am(repo, patch_data, threeway=False, directory=None, exclude=None,
     committer_name=None, committer_email=None, keep_cr=True):
-  args = []
+  # --keep-non-patch prevents stripping leading bracketed strings on the subject line
+  args = ['--keep-non-patch']
   if threeway:
     args += ['--3way']
   if directory is not None: