When rendering a side-by-side diff, `SourceDimensions::new` could compute
a per-column content width that was not greater than the tab width. That
violates the precondition of `split_and_apply`, which asserts
`max_len > tab_width` so it can always fit at least one expanded tab and
make progress when wrapping. As a result difftastic aborted with a panic
on ordinary input, for example `difft --width 14 a b` or, even at the
default terminal width, `difft --tab-width 100 a b`.
A very small `--width` could also drive the total display width down to
two columns or fewer, underflowing the `display_width - SPACER.len()`
subtraction (previously guarded by an assertion that itself aborted).
Enforce both invariants in `SourceDimensions::new`: keep room for the
column spacer, and ensure each content column is wider than the tab
width. Narrow terminals now degrade gracefully instead of crashing.
dualfroz
upstream