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.

autoload/gitgutter/diff.vim

@@ -116,7 +116,8 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
 
     " Write file from index to temporary file.
     let index_name = gitgutter#utility#get_diff_base(a:bufnr).':'.gitgutter#utility#base_path(a:bufnr)
-    let cmd .= gitgutter#git(a:bufnr).' --no-pager show --textconv '.index_name.' > '.from_file.' || exit 0) && ('
+    let cmd .= gitgutter#git(a:bufnr).' --no-pager show --textconv '.index_name
+    let cmd .= ' > '.gitgutter#utility#shellescape(from_file).' || exit 0) && ('
 
   elseif a:from ==# 'working_tree'
     let from_file = gitgutter#utility#repo_path(a:bufnr, 1)
@@ -129,7 +130,8 @@ function! gitgutter#diff#run_diff(bufnr, from, preserve_full_diff) abort
     let cmd .= ' -c "diff.noprefix=false"'
     let cmd .= ' -c "core.safecrlf=false"'
   endif
-  let cmd .= ' diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args.' -- '.from_file.' '.buff_file
+  let cmd .= ' diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args
+  let cmd .= ' -- '.gitgutter#utility#shellescape(from_file).' '.gitgutter#utility#shellescape(buff_file)
 
   " Pipe git-diff output into grep.
   if !a:preserve_full_diff && !empty(g:gitgutter_grep)