Add an external tool selector for diff/merge tool in preferences

[colorful-thundercloud]

Hello,

First of all I wanted to say thank you to all contributors for such an amazing tool!

Been using it for a while, and for merging I prefer Rider over other options, however I can't select it as diff/merge external tool as this setting allows only predefined options. Would be great if there was a way to add Rider and other supported editors as diff/merge tool.
For now Rider window automatically detects if there is merging in process, but sometimes I don't have it opened beforehand and would like to use "merge with external tool" button instead.

[love-linger]

You can configure Rider or any other merge tool not supported by SourceGit in git, and select Use Git Settings in SourceGit.