Stabilization of structuredDiffForFile API

At the moment `structuredDiffForFile` for LocalGit return `chunks` with `ln`, `ln1`, `ln2` and for Bitbucket Server with `destinationLine` and `sourceLine`. Maybe for GitHub another too, I didn't check.

Maybe need to stabilize this API?

Start by locating the structuredDiffForFile implementations for LocalGit and Bitbucket Server, then compare the chunk fields they return. Check the GitHub implementation as well, if present; done means the API contract and consistent chunk field names are defined and applied across supported providers.

Written by the indexing model from the issue text.