chore: typecheck git_message.py

typecheck codemcp/git_message.py

```git-revs
8d3994c  (Base revision)
90d0efb  Add type annotation for revision list variables
0bb8dfd  Add type annotation for more list variables in the old format handling
HEAD     Add type annotation for the final list variable
```

codemcp-id: 224-chore-typecheck-git-message-py
ghstack-source-id: 918cce4
Pull-Request-resolved: #215

Author: ezyang

codemcp/git_message.py

@@ -78,15 +78,15 @@ def update_commit_message_with_description(
         message_after = main_message[end_marker_pos + len(END_MARKER) :]
 
         # Parse the revision list
-        rev_entries = []
+        rev_entries: list[str] = []
         if rev_list_content:
             rev_entries = [
                 line.strip() for line in rev_list_content.splitlines() if line.strip()
             ]
 
         # Process rev_entries: replace any HEAD entries with actual commit hash
         has_base_revision = False
-        new_rev_entries = []
+        new_rev_entries: list[str] = []
 
         for entry in rev_entries:
             if "(Base revision)" in entry:
@@ -137,8 +137,8 @@ def update_commit_message_with_description(
 
         if has_head_entry or has_base_revision:
             # Old format detected, convert to new format with markers
-            new_rev_entries = []
-            filtered_lines = []
+            new_rev_entries: list[str] = []
+            filtered_lines: list[str] = []
 
             for line in lines:
                 if "(Base revision)" in line or line.strip().startswith("HEAD"):
@@ -192,7 +192,7 @@ def update_commit_message_with_description(
         else:
             # No markers or HEAD entries, handle as normal
             if description:
-                rev_entries = []
+                rev_entries: list[str] = []
 
                 # If we have a commit hash, initialize with base revision
                 if commit_hash: