Include git-annexed content in PRs

[JuliaSprenger]

Describe the feature
include git-annex content in PRs

Describe the solution you'd like
When a PR between repositories (forks) is merged, that contains files tracked by git-annex, the content of these files should also be made available in the target repository.

Describe alternatives you've considered
Not working with forks of repositories and instead providing access rights to the repository to the whole gin community. This is not feasible, since 1) it's not possible to list all existing GIN accounts as collaborators and 2) probably leads to a very chaotic repository later on.

Additional context
This request is related to an ongoing discussion in the context of https://gin.g-node.org/NeuralEnsemble/ephy_testing_data on how collaborative work and contributions from non-collaborators can be integrated consistently.