FileMerge is a nice diff and merge tool for OS X, and I use it a lot for larger code reviews where lots of context is helpful. It also supports intra-line diff, which comes in pretty handy.
However in recent releases, at least in v2.8 which comes as part of XCode 6.1, it assumes you want to be merging and shows that bottom pane. Adjusting it away doesn’t persist to the next time you use it, *gnash gnash gnash*.
The solution is to open a terminal and offer this incantation:
defaults write com.apple.FileMerge MergeHeight 0
Unfortunately, if you use the merge pane then you’ll have to do that again. Dear Apple, pls fix!