New items sometimes don't get marked as attachments

[fluffy-critter]

Expected Behavior

Items which are attachments should be attached once they have IDs assigned

Current Behavior

When bulk-importing a bunch of entries, if an item references other items as attachments but those other items have not yet had IDs generated, the Attach directive is ignored.

Possible Solution

When the indexer sees a new item with missing attachments, it should be scheduled for a fixup pass.

Steps to Reproduce (for bugs)

1. Create a bunch of new items, e.g. root.md item1.md item2.md, where root.md has headers like:

   Attach: item1.md
   Attach: item2.md
   

2. Index the new items

Context