fix: fix includig diagonal elements in dense adjacency matrices

[szhorvat]

Fixes #1429

This does fix the issue reported in #1429, and is ready to go right now in that sense. But you may want to polish it up to prepare the ground for future improvements. I leave it in your hands @Antonov548 @krlmlr

You may also want to add some tests (weighted and unweighted undirected case).

[aviator-app]

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[krlmlr]

@maelle: Can you please add a test to this PR? Looks good to me otherwise.

[maelle]

On it now 😸

[maelle]

All test cases except the fourth one fail for the main branch. 🎉

[krlmlr]

Nice, thanks!

[aviator-app]

This pull request failed to merge: PR cannot be automatically rebased, please rebase manually to continue. Once the issues are resolved, remove the blocked label and re-queue the pull request. Note that the pull request will be automatically re-queued if it has the mergequeue label.

Additional debug info: Failed to rebase this PR onto the latest changes from the base branch. You will probably need to rebase this PR manually and resolve conflicts).