SignifyRefresh: use window ID instead of number

`win_getid()` and `win_gotoid()` were introduced in 7.4.1557.

Older versions will continue using the less accurate window number instead.

autoload/sy/util.vim

@@ -24,9 +24,19 @@ endfunction
 
 " Function: #refresh_windows {{{1
 function! sy#util#refresh_windows() abort
+  if exists('*win_getid')
+    let winid = win_getid()
+  else
     let winnr = winnr()
+  endif
+
   windo if exists('b:sy') | call sy#start() | endif
+
+  if exists('winid')
+    call win_gotoid(winid)
+  else
     execute winnr .'wincmd w'
+  endif
 endfunction
 
 " Function: #hunk_text_object {{{1