bug#51757: 27.2; [patch] man.el : wait for all man(1) output to be buffered before fontifying

[Juri Linkov]

>> Of course, with the man.el from emacs-28, I get the error:
>>   'completing-read: Symbol’s function definition is void: format-prompt'
>> under Emacs-27.2 , which is NOT Emacs-28 :-( .
>> 
>> If you can do 'M-x manual-entry bash' in Emacs-28, and
>> it never misses a font transition, let me know - I will
>> re-examine once Fedora's Emacs 28 comes out.
>> 
>> But I enclose a screenshot of how the bash manpage looks with the
>> Emacs 27.2 man.el, with all the remaining text of the man-page
>> after the start of the 'PARAMETERS' section in bold, because
>> only a partial escape sequence was at the end of the buffer
>> and the transition was missed by fontification-on-the-fly .
>
> I just did that, and I see no problems with the font transitions.

Maybe reproducibility depends on the value of 'read-process-output-max'?
But I tried to set it to a very small value (100), and still no ANSI
sequence was emitted partially between font-lock iterations.