Fixes this bug:

    More-prompt is skipped when doing this; (Randall W. Morris, Jun 17)
        :au
        <Space>
        b
        <Space>

The bug can be triggered by every listing that calls do_more_prompt() in
message.c, i.e. also ":version", ":ls", ":highlight", etc.  The reported key
sequence for triggering the bug is far from the only one.

A wrong conditional in the scrolling system of message.c causes the trouble.
When scrolling downwards, the code at message.c:2613-2622 get as many lines
from the scroll list as needed (scroll > 0) or possible (mp_last != NULL).
scroll is the number of lines to scroll down, and mp_last is a pointer to the
scroll list.  This is necessary, as a caller of the message output functions
will only generate its output once.  For that reason, if the user has scrolled
all the way to the bottom, subsequently all lines will be fetched from the
scroll list.  If all required lines were fetched from the scroll list (scroll
has become zero), or the scroll action is upwards (scroll was set to a negative
number in the switch statement), the conditional of the statement at
message.c:2625 should evaluate to true and have the more-prompt displayed
again.  Unfortunately, the event of scroll being zero can only make the
conditional true if we are not at the end of the scroll list (mp_last != NULL),
triggering the bug.  I have corrected the conditional such that it does not
depend on the advancement of the scroll list.

Created by Martin Toft <mt@martintoft.dk> during Google Summer of Code 2007.


Index: src/message.c
===================================================================
*** src/message.c	(revision 343)
--- src/message.c	(working copy)
***************
*** 1878,1884 ****
  		/* output postponed text */
  		t_puts(&t_col, t_s, s, attr);
  
! 	    /* When no more prompt an no more room, truncate here */
  	    if (msg_no_more && lines_left == 0)
  		break;
  
--- 1878,1884 ----
  		/* output postponed text */
  		t_puts(&t_col, t_s, s, attr);
  
! 	    /* When no more prompt and no more room, truncate here */
  	    if (msg_no_more && lines_left == 0)
  		break;
  
***************
*** 2234,2240 ****
  {
      msgchunk_T	*mp;
  
!     /* Only show somethign if there is more than one line, otherwise it looks
       * weird, typing a command without output results in one line. */
      mp = msg_sb_start(last_msgchunk);
      if (mp == NULL || mp->sb_prev == NULL)
--- 2234,2240 ----
  {
      msgchunk_T	*mp;
  
!     /* Only show something if there is more than one line, otherwise it looks
       * weird, typing a command without output results in one line. */
      mp = msg_sb_start(last_msgchunk);
      if (mp == NULL || mp->sb_prev == NULL)
***************
*** 2622,2628 ****
  		}
  	    }
  
! 	    if (scroll < 0 || (scroll == 0 && mp_last != NULL))
  	    {
  		/* displayed the requested text, more prompt again */
  		screen_fill((int)Rows - 1, (int)Rows, 0,
--- 2622,2628 ----
  		}
  	    }
  
! 	    if (scroll <= 0)
  	    {
  		/* displayed the requested text, more prompt again */
  		screen_fill((int)Rows - 1, (int)Rows, 0,