Increase file collapsing threshold when vieweing diffs file-by-file

Description

When I'm reviewing a MR file-by-file, I'd expect to see the whole file like I do in my IDE 😄

In GitLab file-by-file	In IDE

Taken from this MR !42443 (merged)

In this example ☝, I don't know if we are adding a computed or a method, but when I review locally all the context is right there.

Proposal

Let's smartly adjust the threshold of collapsing file content when reviewing file-by-file.