That's interesting. I never use SourceTree to invoke a merge but use ctrl-D to open the external diff tool a lot, in my case that's either WinMerge or Beyond Compare. I also notice WinMerge shows up in external diff tool list but not in external merge tools. Even putting Beyond Compare in the Merge tool configuration yields greyed-out menu as you showed.
I use SourceTree on OS X also, diff and merge tool is FileMerge. The merge menu is greyed out in SourceTree 2.3.1 also.
Conclusion: The function isn't implemented in either version of SourceTree or it's broken in both.
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