SVG tester: Compare SVGs to the parent branch

[sophiamersmann]

Compare SVGs to the parent branch, which is not necessarily the master branch in a stack of PRs. For example, in a stack master –> first-branch –> second-branch, second-branch would be compared to first-branch and first-branch would be compared to master

[marcelgerber]

We agree this would be helpful, but also there is quite some added complexity in regards to checking out the correct commit to start from, and keeping base commits and everything like that in sync while changes are actively being pushed to all involved branches.