Ref T12464. We currently use SHA1 to detect when two files have the same content so we don't have to store two copies of the data.
Now that a SHA1 collision is known, this is theoretically dangerous. T12464 describes the shape of a possible attack.
Before replacing this with something more robust, shore things up so things work correctly if we don't hash at all. This mechanism is entirely optional; it only helps us store less data if some files are duplicates.
(This mechanism is also less important now than it once was, before we added temporary files.)