Provide a subcommand to commit subdags.

[johanneskoester]

Inside a modified branch, you can now run e.g.

bioconda-utils commit-subdags recipes/ config.yml -k 1 -n 10 --msg "Fixing bioconductor datapackages"

Bioconda-Utils will then create a commit with the k-th of n subdags of changed recipes, properly considering dependencies. @bioconda/core, this could be a potential replacement for the bulk branch in the future. Let me know your thoughts. I used this to create PR bioconda/bioconda-recipes#13946.