SourceTree for Windows: No options for external diff/merge

Question

I am trying to setup SourceTree to use Araxis Merge for external diff and merge. The problem is that under Tools->Options->Diff, the only option available is "System Default". I cannot click the dropdown to see other options, it appears to be disabled.

I am using SourceTree for Windows 1.3.2.

nadnevi · Accepted Answer

I ran into this today - you have to check the "Allow SourceTree to modify your global Git and Mercurial config files" option on the General tab.  Then you should be able to change the Diff settings - you can even uncheck the "Allow SouceTree to modify..." option when you're done setting your diff program.

I had thought that the External Diff settings were just for SourceTree, but apparently they are actually a GIT option (there is a git difftool command), which is why you have to allow SourceTree to modify your settings to be able to change them.