test: assert exact CRLF diagnostics

[PandaHUN777]

Summary

• Assert the exact diagnostic for CRLF commit messages read by --hash, --from-hash, and --file.
• Update the command-line newline normalization note to match current behavior.
• Add a lone-CR --file regression that detects removal of file newline normalization.

Closes #32

Validation

• cargo +stable build
• cargo +stable test
• cargo +stable fmt --all --check
• cargo +stable clippy --all-targets --all-features -- -D warnings
• git diff --check
• Confirmed the lone-CR test fails when file newline normalization is removed.