Move the FileDiffData insert/delete line counts into extra_data.
This replaces the insert/delete line count fields with entries in a new

extra_data field. These count fields were introduced in 2.0 beta, and

long-term, we may want additional metadata around diff data, so it's a

good time to replace them.
This will introduce another potentially long evolution, given that there

may be a lot of entries in the database. However, it'll help prevent

long evolutions in the future, and won't be any longer for users who

upgrade from 1.7.x or earlier (since either way, they'll need to evolve

this table). Might actually be faster, since there's only one column to

add.

Evolutions and unit tests passed.