MR refactor: `diffFile` is being stored twice in Merge Request store with 2 different formats snake_case and camelCase

The diffFile object is being stored when SET_DIFF_DATA mutation is commited.

It will store the diff_file with converted props from snake_case to camelCase.

However, the diff_file is already stored under the rootState for each discussion in discussions, and it's being stored with snake_case properties

We only need a few props under the discussions (file id, line number), we could increase performance if we didn't store it so many times, the diff_file object is huge.

/cc @timzallmann @andr3