Maybe this will help others. I installed Git from https://git-scm.com/download/win then I copied git.exe from C:\Program Files\Git\bin to c:\windows\ and I saw (in Procmon) that Fritzing stopped trying to find Git and then moved on to tell me there were some old files to clean, then it offered the parts update, and off it went. No problem opening my existing fzz files, no corruption.
But the underlying problem is that Fritzing is trying to find git.exe (and git.cmd, and some other git -related files). Hopefully this problem can be fixed in the next Fritzing release. 