That's because it's not obvious how to do it
There is no built-in action that is run every update cycle, so you have to make it so that some ifAbove/Equal/BelowAction is triggered every update cycle:
You have to set your Powershell execution policy to "RemoteSigned" or "Unrestricted" (I'd choose the former), and set the path in the first line to be where you keep your repos. You can make the path "C:\" to crawl your whole drive, but that takes a while. The file ends up in your temp directory.
Like a lot of scripting languages, this will pop up a console window when you run it. So, the best way around it is to execute it from a VBS with "powershell -file ScriptName.ps1" and making the window hidden. I'm still working on the part where Rainmeter runs it and displays it, but I think this answers your question. Note that this only looks for Mercurial or Git repos. If you're using some other vcs... switch to Mercurial or Git.