fix: truncated message with emoji

[ColonelBucket8]

Fixes #495

• Switch to UnicodeWidthStr::width; truncation now walks
  graphemes accumulating cell width with 1-cell ellipsis reservation in
  cell units.
• Move unicode-width from [dev-dependencies] to
  [dependencies].

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md