code_style: general cleanup

[NathanBaulch]

Various code improvements. The changes to QueryLocalChanges in particular make the code much easier to understand.

[love-linger]

I no longer plan to merge this kind of PR.

1. There are too many code changes, which takes too much time to review.
2. Sometimes I can't recall the issues that needed to be considered during the coding process, resulting in bugs, such as 2025.26 Can't UNSTAGE files from stage when checked Amend checkbox #1582, 2025.26 doesn't startup if the path has a space in it. #1576, d28bd92, etc.
3. Most of them are about new syntax or variable initialization suggestions.

[NathanBaulch]

No worries, I only submitted this because you were so quick and thorough at reviewing previous cleanup PRs. I wasn't aware there had been regressions.