Respect width in ansi_collapse(style = "head") when no element fits

[taekop]

ansi_collapse(style = "head") can return a string wider than width when not even the first element fits. In that branch the first element was trimmed to width and the ellipsis was then appended, so the result was width plus the ellipsis width.

x <- c("loooooooong", "healthy live the Penguin")
ansi_collapse(x, width = 2, style = "head")
#> "lo…"   (3 wide)
ansi_collapse(x, width = 3, style = "head")
#> "loo…"  (4 wide)

The element is now trimmed to width - ellipsis width, giving "l…" and "lo…". Output that already fit within width is unchanged.

Closes #570

[gaborcsardi]

Thanks!