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.

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

That said, I tested the changes locally in my project by building gatsby-link manually.
For what it's worth, this change didn't add any more test failures than I already had.
I hope the CI can build this.

Documentation

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

Related Issues

Fixes #22124

[abarisain]

I'll have a look at why the tests are failing