Identifier of the earlier commit that contains the item being extended.
Together with commitIdentifier, this identifies the exact item to extend when a later
commit supplies information that was not added earlier. For example, when an input was
already provided without unlockingBytecode, a follow up commit can add a new input
entry with mergesWith, itemIndex and unlockingBytecode. The merger then uses this index to
locate that earlier input and populate the missing field without replacing the commit.
This variable is named 'item' because it's used with variables, inputs and outputs.
Note: The index starts at 0 for the first item in the commit data array.
Reference to an item in an earlier commit that a variable, input, or output extends.