Example reproduction steps:
1. Open several files in one copy of vim; for example: vim *.c
2. While the first copy is running, open the same files in another copy of vim; 
in another terminal window, for example.
3. Hit "o" at each swap file prompt, and the spacebar at each "-- More --" 
prompt.

The "-- More --" prompt currently shows up far too often, about every three 
files on a large screen.  I expect the swap file prompt to count as having 
allowed me to see the lines on the screen.

This occurs on every copy of vim I've tried; in particular, it happens in 
Ubuntu 10.10's pre-packaged 7.2 (2:7.2.330-1ubuntu4), both gui and console, and 
in hand-built console versions from the Mercurial repository: vim72 branch 
(v7-2-446) and default branch (v7-3-230).

The attached patch attempts to fix the issue for all console prompts, by 
resetting lines_left in msg_end_prompt().  This has been verified to fix the 
swap file prompt for both Mercurial heads, but has not been tested for other 
types of console prompts, nor for gui prompts.