--commit-style box and/or --file-style box causes write_boxed_with_line to panic

[nkouevda]

With a narrow terminal (<= 48 chars), --commit-style box results in:

> git show
thread 'main' panicked at 'attempt to subtract with overflow', src/draw.rs:39:35
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Because line_width <= box_width, so line_width - box_width - 1 would be negative.

Or a more practical scenario: even with a reasonably wide terminal, very long filenames cause the same issue under --file-style box.

[dandavison]

Thanks @nkouevda! These should be fixed in v0.0.13