fix(blame): avoid underflow when blaming an empty file

[Noethix55555]

When an empty file has been committed, git blame succeeds and returns 0 lines. Two sites in blame_file.rs compute lines().len() - 1 without guarding against the empty case, causing a usize underflow: panic in debug builds, usize::MAX in release.

Fixed both get_max_line_number and the goto-line event handler to use saturating_sub(1).