aft: Generate a remove event when adding duplicate files.

aft: Generate a remove event when adding duplicate files.

Consider the following scenario:

	add /foo.mp3
	add /bar.mp3 # assume foo and bar differ
	cp /foo.mp3 /bar.mp3
	add /bar.mp3

In the last add command, the path being added (/bar.mp3) already exists
in the audio file table, and its hash also exists for a different path
(/foo.mp3).

The code in aft.c is smart enough to detect this. It first removes
the existing entry for /bar.mp3 and then considers the addition of
/bar.mp3 as a file rename /foo.mp3 -> /bar.mp3.

Unfortunately, we miss to generate the remove event in this case. This
patch should fix it. It also improves the message a bit.