update batching

[thepwagner]

This adds support for update batching, which allows arbitrarily combining updates in PR branches.
Previously all updates were pushed to unique branches, and there was some tested but unintegrated code to combine all updates into a single PR.

Batches have names, which are used in the created update branches.
Batches are defined by a set of prefixes for package paths they should include.
The following sample defines a batch named internal that will combine all updates to my own modules.

      - uses: thepwagner/action-update-go@batching
        with:
          batches: |
            internal: github.com/thepwagner

In tackling this I moved branch naming to the RepoUpdater's scope, letting the Repo focus on dumb git operations.
That took with it the path for Repos to enumerate open/closed PRs; which may come back?

[thepwagner]

If this were a product, I'd miss ignoring closed updates.

The problem is parsing the branch name to determine the ignored dependency falls flat when batching means branch names are no longer indicative of the dependency updated.

I think they come back once they commit message (or maybe PR body?) start including more metadata about the updates.

[thepwagner]

This took ~10 minutes of WTF to rediscover. The slow kind where you have to push + trigger a workflow each iteration, and everything "just works" outside of Actions.

Tracked in actions/runner#665