gitk external diff on Windows

I'm running git version 2.28.0.windows.1 on Windows and trying to set it up to run winmerge as the external diff tool. I am pretty sure I used to do this following the instructions here:

https://igorsgames.wordpress.com/2015/05/28/using-winmerge-as-an-external-diff-tool-in-gitk/

When I point gitk to the batch file and try to open the external diff tool, nothing happens. Winmerge does not open, nor do I get an error from gitk. I noticed the next box below on the preferences screen contains "cmd /c" so I tried prefixing the path to the batch file with that, and instead I get an error that says:

cmd /c C:/Program Files/WinMerge/winmerge.bat: command failed: couldn't execute "cmd \c C:\Program Files\WinMerge\winmerge.bat": no such file or directory.

Why does nothing happen in the first case, and why in the second case is gitk apparently changing regular slashes into back-slashes before trying to pass the string directly to CreateProcess as the name of the executable binary, rather than to a shell or ShellExecute or something?

Also I just tried having the batch file echo something to a text file to see if it was running at all or if winmerge just isn't opening, and the text file ( full path to my home directory to make sure permissions aren't a problem ) isn't created, so the batch file isn't executing even though there is no error given by gitk.