Use shellescape on paths in run_diff

In Neovim 0.11.1 within MSYS2 on Windows, gitgutter wasn't showing up.
After some flailing around, I found adding
`set shellcmdflag=-c shellxquote= shellxescape=`
[from here](https://github.com/neovim/neovim/issues/28384#issuecomment-2135921829),
got `:echo system('git --version')` working. The signs were still not
showing up, but there were files being dumped in my current working
directory that looked like corrupted Windows paths.

After using `let g:gitgutter_log=1` and looking at the log I found where
these temp files were made and I think I fixed it. I haven't tried the
non-msys2 Neovim (assuming that's different) or normal vim, but I
brought the change over and made sure it worked in Neovim on Linux.