bug#65554: 30.0.50; help-echo string for truncated tab-bar names is not viewable

[Juri Linkov]

> If you set tab-bar-tab-name-function to use tab-bar-tab-name-truncated,
> the latter function sets the help-echo string of the tab-bar name to the
> full name

Actually the help-echo string with the full name is not viewable
even if you remove the :help lines from tab-bar--format-tab.

I propose to move truncating the name from
tab-bar-tab-name-function to
tab-bar-tab-name-format-function.

tab-bar-tab-name-format-function was added later, so initially
it was possible to implement truncation only in
tab-bar-tab-name-function.

> however, tab-bar--format-tab which is used by
> tab-bar-format-tabs overrides this help-echo string with "Current tab"
> and "Click to visit tab".  If I remove :help plist items from
> tab-bar--format-tab, then I don't get the help-echo string that was set
> by tab-bar-tab-name-truncated but what the tab looks like in Emacs.

When truncation will be in tab-bar-tab-name-format-function
then the useless help-echo strings in tab-bar--format-tab
could be replaced with help strings that display the full names.