filediff_creator.create_filediffs is now split into:

filediff_creator.prepare_filediffs, which does the work that

create_filediffs used to do up to actual creation (with

FileDiff.objects.bulk_create().
filediff_creator.finalize_fildiffs, which takes the result of

prepare_filediffs and does the actual creation.

In the new DVCS web API workflow, we will need to validate that the

created FileDiffs from the cumulative diff match (a) the validation info

passed and (b) the set of terminal FileDiffs. If they do, we will

proceed with creation. This allows us to finalize in a single API call

instead of two (as before we would have to call create_filediffs()

with validate_only=True to validate and then again with

validate_only=False only when validation has succeeded. Since this

requires us to parse the entire diff file again, we would not

necessarily want to do it in the same API request.

Ran unit tests.