How do you exit vimdiff mode in vim, specifically, for Fugitive?

You can execute windo set nodiff noscrollbind and then close the second window.

Update: there is a diffoff command. Use windo diffoff, not what I wrote in previous line.

According to: https://github.com/tpope/vim-fugitive/issues/36

Close the other window. The easiest way to do this if you haven't shifted focus to it is <C-W><C-O>, which means "make this Window the Only window."

I had no luck with diffoff, but I just learned that :Gedit with no argument will bring you back to the working-directory version of the file, as opposed to some earlier version you were reviewing.

And as q (no need for :q) will close the diff sidebar, you can do q followed by :Gedit to get rid of the sidebar and then go back to the current version of the file.