Adding mpm (Matt Mackall) to nosy.

@GrayNM:

If a broken virus scanning process is holding on files such that regular 
programs can't access them any more, we are lost. Don't use such stupid 
scanners in the first place.

You might want to take this up with the makers of the broken scanner.

Adding workarounds of the sort of "let's see if we are allowed to write to 
this file now and try again later if not" for such broken products to 
mercurial doesn't make any sense.