Instagram / IGListKit / 5489555151

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fix crash when calling reloadObjects during an update

Summary:
Follow up to D47263265

## What's happening
We're calling `listAdapter.reloadObjects(...)` in the middle of the IGListKit's internal data update, so it's using the new section index path, instead of the old one. So we're going from a list of 2, insert at 0, and try to reload index 1, then we crash (it'll try to update index 2, which doesn't exist in the old array)

## Fix
Just like for `[IGListAdapter performBatchAnimated:updates:completion:]`, lets use the old index. To get this working, lets ask the updater if we're in an update, rather than `IGListAdapter` try to keep track of it.

Sorry the diff is a bit long, but in case this feels reasonable to pick, I might

Differential Revision: D47281472

fbshipit-source-id: 193153cfc

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