diff: -y --tabsize=<huge> overflows the column arithmetic (overflow-checks only) #264

SideDiffConfig::new (src/side_diff.rs) computes the side-by-side column layout with signed arithmetic on the --tabsize value:

let w = full_width as isize;
let t = tab_size as isize;
let t_plus_g = t + GUTTER_WIDTH_MIN as isize;      // <-- side_diff.rs:85: overflows
let unaligned_off = (w >> 1) + (t_plus_g >> 1) + (w & t_plus_g & 1);
let off = unaligned_off - unaligned_off % t;       // (also % by t, and t can be 0)

--tabsize is parsed into tab_size: usize with no upper bound. A value near isize::MAX makes t + GUTTER_WIDTH_MIN (GUTTER_WIDTH_MIN == 3) overflow isize. Under -C overflow-checks=on this aborts (exit 134); in the default release build it wraps to a bogus value and the layout math silently produces garbage widths.

$ printf 'a\n' > f1; printf 'b\n' > f2
$ diff -y --tabsize=9223372036854775805 f1 f2      # overflow-checks build
thread 'main' panicked at src/side_diff.rs:85:24:
attempt to add with overflow
$ echo $?
134