fix: respect platform line endings in rustfmt

[CAOShurong]

This Pull Request fixes/closes#1936.

It changes the following:

• lets rustfmt preserve the checkout's native line endings instead of forcing Unix newlines on every platform;
• keeps LF behavior on Unix-like checkouts while allowing a normal Windows core.autocrlf=true checkout to pass the repository's format check.

I followed the checklist:

• I added unittests (not applicable to this rustfmt configuration change)
• I ran make check without errors (the full default-feature build needs a complete Perl/OpenSSL toolchain that is not present in this Windows environment)
• I tested the overall application (no runtime code changes)
• I added an appropriate item to the changelog (no user-facing behavior changes)

Verification on Windows with core.autocrlf=true:

• the original newline_style=Unix behavior exits 1 with 162 Incorrect newline style findings;
• cargo fmt -- --check exits 0 with this change;
• a separate clean core.autocrlf=false clone reports index/worktree LF and also passes cargo fmt -- --check without changes;
• cargo nextest run --workspace --no-default-features --features ghemoji,regex-fancy,trace-libgit passes all 313 tests;
• git diff --check passes and the worktree remains clean after formatting.

The change and this description were prepared with OpenAI Codex assistance. I independently reproduced the issue and checked the commands and results above.