Extend CheckView to behave more like the actual merge. Now the merged file is also written to a temporary file in order to make sure the merge object actually reads all the contents of the input file. As it turns out, some contents of the input file are only read on demand and simply appending the file to the merge does not trigger this. Part of mpuls/issue5709

bd3e5fdd839808907987993e960e7f68810ec951 1.0
9ad8421dafb43c2df9c11472b51d575c0553aa27 1.1
c018570c1a2d300bededc7aea5a311bb822f4600 1.2
4f45ebecb057ec4ab675b56731a816fbb6692c32 1.3
74ba8c361d667b0d417e830e2328833b4296af79 1.4