fix(gatsby-link): replace current path in history rather than pushing it

[abarisain]

Hello!
I'd like to start by thanking you for this awesome project.
Sorry if this contribution doesn't match the guidelines, but I hope I understood them well.

Description

This change makes <GatsbyLink> ask navigate to use replaceState rather than pushState when navigating on a link that is the current page. This fixes an issue where a history item would be pushed multiple times for the current page, while this is not the expected behaviour.

If a developer forced "replace" using the prop, their choice takes priority.

This uses the same technique as @reach/router when they fixed it upstream: reach/router@11e9ed6

I did not copy the

const { key, ...restState } = { ...location.state };
shouldReplace = shallowCompare({ ...state }, restState);

part, because I do not understand it.

I also added some tests.

Note: I did not manage to get unit tests to run on my computer (either by using Node 10, 12, 13 or 14, or even trying on all 3 operating systems). They didn't work in the linked gitpod either.

That said, I tested the changes in gatsby-link using yarn jest in its folder.

Documentation

I don't believe this needs documentation other than updating the changelog

Related Issues

Fixes #22124

[blainekasten]

This looks great! I tested out the behavior and it works as I would expect! Thanks for the contribution

[gatsbot]

Holy buckets, @abarisain — we just merged your PR to Gatsby! 💪💜

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Thanks again!

[abarisain]

Thanks! I'm glad to have been able to contribute :)