For anyone else trying this, be very careful with what you type and which directory you are in when you type it, the above could delete your entire app and its rewind history if you happened to be in the root directory.
A more precise version here, note that it will delete your rewind history:
rm -rf /app/.git/
@cori This topic really should be in the FAQ or at least in Glitch: The friendly community where everyone builds the web somewhere, and cover the usual scenarios that cause it, separation using .data, the garbage collection etc.