Inter-branch merge does not support multiple flows from the same source branch

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

In roslyn, we want to merge `main` into both `release/dev18.0` and `main-vs-deps`. The [inter branch merge config](https://github.com/dotnet/arcade/blob/e52018af7f5bab97b0721382d52badfc7edb54f6/Documentation/Maestro/New-Inter-Branch-Merge-Approach.md) does not seem to support that - the source branch is the key in the JSON, so only the last one will be preserved. I can work around that by running the merge multiple times (separately for each "duplicate" flow): https://github.com/dotnet/roslyn/blob/eb565d86118ab999809925bb5253527708cf460d/.github/workflows/main-merge.yml#L20-L30