Only try to detach UFO if it is loaded

Without this check, it could be that ufo.setup() is not called before
ufo.detach(), which breaks UFO's internal assumptions, and causes an
error.

lua/neogit/lib/buffer.lua

@@ -388,10 +388,12 @@ function Buffer:show()
   end
 
   -- Workaround UFO getting folds wrong.
-  local ok, ufo = pcall(require, "ufo")
-  if ok and type(ufo.detach) == "function" then
-    logger.debug("[BUFFER:" .. self.handle .. "] Disabling UFO for buffer")
-    ufo.detach(self.handle)
+  if package.loaded["nvim-ufo"] then
+    local ok, ufo = pcall(require, "ufo")
+    if ok and type(ufo.detach) == "function" then
+      logger.debug("[BUFFER:" .. self.handle .. "] Disabling UFO for buffer")
+      ufo.detach(self.handle)
+    end
   end
 
   self.win_handle = win