bug#38693: 27.0.50; tab-bar: automatic tab names should be shortened

[Juri Linkov]

>>>> Please provide an option to dynamically shorten at least
>>>> automatic tab labels in order not to overflow the tab bar into
>>>> another line OR provide an option with sensible default for the
>>>> max length of automatic tab labels (e.g. 23 characters per tab
>>>> label would allow for at least 3 tabs on a 80 char terminal) OR
>>>> something to such effect.
>>>
>>> Maybe such a new option:
>>>
>>>   (defcustom tab-bar-tab-name-max 23
>>>
>>> and using it on the default values of tab-bar-tab-name-current,
>>> tab-bar-tab-name-current-with-count, tab-bar-tab-name-all?
>>
>> yes, this would be helpful for me.
>
> I'm still not sure whether to use the new option in all functions above,
> or add a new function that truncates the name.  It seems the latter is
> better:
>
> +(defcustom tab-bar-tab-name-truncated-max 20
> +  "Maximum length of the tab name from the current buffer.
> +Effective when `tab-bar-tab-name-function' is customized
> +to `tab-bar-tab-name-truncated'."
> +  :type 'integer
> +  :group 'tab-bar
> +  :version "27.1")

The same option should be added also to tab-line to truncate tab-line
tab names.  Currently I fixed a bug that occurred when a tab name
was longer than window width.  With such an option set to some reasonable
default value this situation won't happen.