chore: typecheck git_message.py

[ezyang]

Stack from ghstack (oldest at bottom):

• -> chore: typecheck git_message.py #215
• chore: initialize project for configuration file typecheck #214
• fix: typecheck common.py and fix type errors #213
• fix: resolve type checking errors in code_command.py #212
• fix: typecheck and fix errors #211
• fix: typecheck and fix errors in glob.py #210
• fix: typecheck line_endings.py and create editorconfig stub #209

typecheck codemcp/git_message.py

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