bug#24193: 25.1; `window-min-size' fails for horizontal width when margins >= body text

[Paul Rankin]

There appears to be a change to `window-min-size' that gives an erroneously 
large minimum horizontal window size when the combined size of window margins 
exceed that of the body text, e.g. in a frame 80 columns wide, with left margin 
20 columns and right margin 20 columns, would return a minimum horizontal size 
of 42, when body text would be 40. This causes `split-window-right' to fail.

To reproduce:

1. emacs -Q
2. in a frame 80 columns wide...
3. M-: (set-window-margins nil 20 20)
4. C-x 3
    => "Window #<window 1 on *scratch*> too small for splitting (2)"
5. M-: (window-min-size nil t)
    => 42

Expected results:

Window should split horizontally. Minimum window width should not be dictated 
by margin width.

Actual results:

Margin width dictates minimum window width and prevents window splitting.

Configuration:

GNU Emacs 25.1.1 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 10.11.6 
(Build 15G31)) of 2016-08-09

MacBook Pro (Retina, 15-inch, Mid 2014)
2.2 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel Iris Pro 1536 MB