When b:sy is present, only disable Sy if the file disappears

In mhinz#344, we disable Sy if any condition in s:skip() is true.  This is
too aggressive.  If b:sy was already set then we should prefer to leave
it set, except when we can't access the file anymore.

Therefore, stop using s:skip() to handle this condition and simply check
filereadable().

Closes mhinz#347

Author: jamessan

autoload/sy.vim

@@ -42,7 +42,7 @@ function! sy#start(...) abort
     return
   else
     let path = s:get_path(bufnr)
-    if s:skip(bufnr, path)
+    if !filereadable(path)
       call sy#stop()
       return
     elseif empty(sy.vcs)