bug#17831: 24.4.50; bad default value for `Man-width'

[Juri Linkov]

>> Man-width defaults to the window width at the time of running `man'.  But
>> if the frame is split horizontally it usually leads to a view with the
>> right-hand-side half unviewable.
>
> Indeed.
> As discussed earlier, I think the right fix is to display the buffer
> right away and then fill it in the background.
>
> Problem with it, is that currently we do "fill the buffer with
> half-processed text; finish formatting; display", so if we only move the
> "display" part, the user will see (temporarily) the half-processed text.

While testing the changes for bug#17809 I noticed a similar problem for
the *Completions* buffer.  It's filled with completions using the
default width of 80 columns while the buffer is not yet displayed.
Later when the buffer is displayed in a window with the frame's full-width
of 160 columns, it leaves too much empty space in the buffer.

As a general solution for such cases it seems the proper order is
to display an empty buffer first, and then fill it with the contents.