feature request: Diff mode

[alexmozaidze]

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Describe the feature

Diff mode - a feature that would highlight changes in oil buffers.

Provide background

I just thought it would be a pretty nice thing to have, though, it's going to be very not so easy to implement (there'll be a lot of edge cases).

What is the significance of this feature?

nice to have

Additional details

No response

[stevearc]

A decent place to start could be leveraging statuscolumn to add signs for new/modified entries. That should be pretty straightforward using the get_entry_on_line function.