Include git-annexed content in PRs

**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.

No files, tests, or implementation entry points are named. Start by locating the fork-to-target pull-request merge flow and its handling of git-annex-tracked files; done means annexed content from a merged PR is available in the target repository.

Written by the indexing model from the issue text.