This is a breaking API change (not that we care about that at this point). This can avoid redundant work spent on atomizing a file multiple times. There are use cases where one particular file must be compared to other files over and over again, such as when blaming file history. The old API gave access to both versions of the file to the atomizer just in case a future atomizer implementation needs this. This can still be achieved by passing a second file via the atomizer's private data pointer.