External Tools can't find TortoiseMerge

I have TortoiseGit installed on my windows machine and want to use it as my diff tool. When I go to Unity Preferences -> External Tools I get "No supported Asset Server diff tools were found" in both Unity3d 5.3.5f1 and 5.5.0f3:

I tried running TortoiseGit repair and restarting my machine. That didn't have any effect.

I do have a custom path for where TortoiseGit is installed. My systems path variable looks correct:

 PATH=C:\ProgramData\Oracle\Java\javapath;C:\Program Files\Microsoft MPI\Bin\;C:\Program Files\Dell\DW WLAN Card;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;c:\Program Files\WIDCOMM\Bluetooth Software\;c:\Program Files\WIDCOMM\Bluetooth Software\syswow64;C:\Program Files (x86)\GtkSharp\2.12\bin;C:\Program Files\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files\dotnet\;C:\Program Files\Microsoft DNX\Dnvm\;C:\Program Files (x86)\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files (x86)\Microsoft SQL Server\130\DTS\Binn\;C:\Program Files\Microsoft SQL Server\130\DTS\Binn\;C:\Program Files\Microsoft SQL Server\Client SDK\ODBC\130\Tools\Binn\;C:\Apps\Dev\Git\cmd;C:\Apps\Dev\TortoiseGit\bin;C:\Apps\Dev\GitLFS;C:\Users\Shawn\AppData\Local\Microsoft\WindowsApps;C:\Apps\Dev\Git\bin;C:\Apps\Dev\Git\usr\bin;C:\Apps\Dev\Git\mingw64\libexec\git-core

Is there anywhere I can manually select the path to the external merge tool?