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.

[daler]

@johanneskoester, revisting this after a while . . .

Nowadays the bulk branch builds separate subdags which, if I understand the rationale for this PR correctly, satisfies the same immediate need as this PR.

In terms of replacing bulk altogether, I kind of like that idea just because it's one less moving part to document and worry about. However an advantage of bulk as it stands now is that it builds on GitHub Actions while everything else is on Azure, so it's on a different queue and quota. It would be good for a bulk replacement to be able to run on a different queue.

That could be handled here by adding special commit strings to signal that it should be built elsewhere -- and then Azure would have to not pick it up and GitHub Actions would need to pick it up. But I feel like that would introduce more complexity than the bulk solution we currently have which simply depends on branch name.