Make fold levels be buffer local.

Without this I had behavior where doing `SignifyFold!` on a second
buffer would change the fold regions of the first buffer. With the
fix things were independent as they should be.

Author: algmyr

autoload/sy/fold.vim

@@ -2,7 +2,7 @@
 
 " SignifyFoldExpr {{{1
 function! SignifyFoldExpr(lnum)
-  return s:levels[a:lnum]
+  return b:levels[a:lnum]
 endfunction
 
 " SignifyFoldText {{{1
@@ -49,7 +49,7 @@ function! sy#fold#enable(do_tab) abort
   endif
 
   let [s:context0, s:context1] = get(g:, 'signify_fold_context', [3, 8])
-  let s:levels = s:get_levels(s:get_lines())
+  let b:levels = s:get_levels(s:get_lines())
 
   setlocal foldexpr=SignifyFoldExpr(v:lnum)
   setlocal foldtext=SignifyFoldText()