bug#22919: calc should use square root symbol to denote square root

[Lars Ingebrigtsen]

Glenn Morris <rgm@gnu.org> writes:

> Alexis wrote:
>
>> to suggest this patch not be applied without also either adding a way
>> to customise which character gets used as the square root symbol, or
>> logic to only display a square root glyph when it's certain it's
>> available.
>
> See eg ruler-mode-fill-column-char and how it uses char-displayable-p.

There was then some worry whether that would work on ttys...  but I
think that's perhaps a too-obscure use case to hinder this change, so I
went ahead and applied the patch (with Glenn's suggestions) to Emacs 28.

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