autocmd: replace BufRead -> BufEnter

This PR replaces the `BufRead` autocmd with `BufEnter.
Closes aymericbeaumet#14.
See me comments on that issue.

In addition to `BufEnter`, I also tried several other autocmd events:
- `BufWinEnter`
- `BufEnter`
- `BufNew`
- `BufAdd`
- `BufWinEnter`
All of these other attempts failed with various incorrect behavior or errors (some of which originated from other neovim plugins that I have installed).

plugin/symlink.vim

@@ -35,5 +35,5 @@ endfunction
 
 augroup symlink_plugin
  autocmd!
- autocmd BufRead * nested call s:on_buf_read(expand('<afile>'))
+ autocmd BufEnter * nested call s:on_buf_read(expand('<afile>'))
 augroup END