ssh/terminal: fix GetSize on Windows

Return window size instead of buffer size.

Fixes golang/go#27743

Change-Id: Ib1cd249f5680d86d505032e51d9102c2718ddf6f
Reviewed-on: https://go-review.googlesource.com/c/163538
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

util_windows.go

@@ -64,13 +64,15 @@ func Restore(fd int, state *State) error {
 	return windows.SetConsoleMode(windows.Handle(fd), state.mode)
 }
 
-// GetSize returns the dimensions of the given terminal.
+// GetSize returns the visible dimensions of the given terminal.
+//
+// These dimensions don't include any scrollback buffer height.
 func GetSize(fd int) (width, height int, err error) {
 	var info windows.ConsoleScreenBufferInfo
 	if err := windows.GetConsoleScreenBufferInfo(windows.Handle(fd), &info); err != nil {
 		return 0, 0, err
 	}
-	return int(info.Size.X), int(info.Size.Y), nil
+	return int(info.Window.Right - info.Window.Left + 1), int(info.Window.Bottom - info.Window.Top + 1), nil
 }
 
 // ReadPassword reads a line of input from a terminal without local echo.  This