Another usrmerge complication

Hi,

because life isn't hard enough as it is: When /bin is a symlink to 
usr/bin, and I install two packages, where one installs /bin/foo and the 
other installs /usr/bin/foo, then, if both are installed in the same 
dpkg invocation, the contents of the first package end up being 
installed, while in two separate invocations, the second package's 
contents win.

So for any undetected file conflict, which file now exists now depends 
on whether apt decided to install both packages during the same 
installation round or not.

Writing more test cases,

    Simon