fixed nasty data loss bug

I wanted to try to guard against it in Command.Add too, but it's a case of
garbage in, garbage out. Once Command.Add has been told it's dealing with a
dummy symlink, it goes and deletes it, and even though the object it
thinks it points to is not present in the annex, it's Command.Add is still
doing the right thing to go ahead and add the broken symlink. So the two
fixes I was able to put in will have to do.

doc/bugs/git_annex_add_removes_file_with_no_data_left.mdwn

@@ -94,3 +94,5 @@ lrwxrwxrwx 1 andrew andrew       195 Jul 12 02:20 Expressionlessm.tar -> ../.git
 
 # End of transcript or log.
 """]]
+
+> [[done]]; this bug is now prevented on several levels. --[[Joey]]