Use the cherry picked CL's Change-Id

If a CL and its cherry pick share a change ID, Gerrit will link to each
other in the UI.

Bug: 364692466
Change-Id: I6740ddfd813124d5876b29b05c4b8f91822faa57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5838865
Auto-Submit: Gavin Mak <gavinmak@google.com>
Reviewed-by: Matt Jones <mdjones@google.com>
Commit-Queue: Matt Jones <mdjones@google.com>

git_cl.py

@@ -4512,8 +4512,11 @@ def _create_commit_message(orig_message, bug=None):
                    "Original change's description:\n")
     for line in orig_message_lines:
         new_message += f'> {line}\n'
+    new_message += '\n'
     if bug:
-        new_message += f'\nBug: {bug}\n'
+        new_message += f'Bug: {bug}\n'
+    if change_id := git_footers.get_footer_change_id(orig_message):
+        new_message += f'Change-Id: {change_id[0]}\n'
     return new_message
 
 

tests/git_cl_test.py

@@ -5451,6 +5451,8 @@ Original change's description:
 > 
 > Bug: 123456
 > Change-Id: I25699146b24c7ad8776f17775f489b9d41499595
+
+Change-Id: I25699146b24c7ad8776f17775f489b9d41499595
 """
         self.assertEqual(git_cl._create_commit_message(orig_message),
                          expected_message)
@@ -5475,6 +5477,7 @@ Original change's description:
 > Change-Id: I25699146b24c7ad8776f17775f489b9d41499595
 
 Bug: {bug}
+Change-Id: I25699146b24c7ad8776f17775f489b9d41499595
 """
         self.assertEqual(git_cl._create_commit_message(orig_message, bug),
                          expected_message)