Witch.sol#L586
In the case where elapsed == duration, proportionNow evaluates to 1e18, which is the same result when elapsed > duration. Proof below.
Of slightly greater importance, this handles the edge case when elapsed = duration = 0, ie. the liquidation transaction is included in the same block / has the same timestamp as the auction initialization transaction
As per the TLDR.
Since the proportion given for zero duration auctions is 1e18, it is equivalent to an auction of infinite duration with 100% initial offer: duration == type(uint32).max and line_.initialOffer = 1e18.
