Badge request: GitHub activity, per branch

When I'm in the middle of a large re-write on a project, I'd really like to have an obvious “activity is happening ” indicator for the README on the stable / master / production branch. I think a shield could serve this purpose admirably.

There's a couple possibilities I see here, any of which (or all!) could be best implemented:
1. Showing date of the latest commit to the project and/or to that branch, i.e.
, , , or , :
`https://img.shields.io/github/activity/USER/REPO.svg`
`https://img.shields.io/github/activity/user/repo/BRANCH.svg`
2. Showing the name of the branch with the most (or most recent?) activity: i.e. , , etc etc:
`https://img.shields.io/github/active-branch/USER/REPO.svg`
`https://img.shields.io/github/active-branch/user/repo/BRANCH.svg`
3. Showing the number of ‘active’ branches, either by a time-range, or by whatever heuristic GitHub uses for the branches page (if that's exposed): i.e. , , etc etc:
`https://img.shields.io/github/active-branches/USER/REPO.svg`

(Could even combine `1.` and `2.` , or `2.` and `3.`  although those are getting a little long!)

Start by reviewing the three proposed GitHub badge URL forms in the issue and decide which activity behavior and branch semantics are in scope. Done means one clearly selected badge design is implemented with the corresponding GitHub data, documented usage, and verification for the expected activity states.

Written by the indexing model from the issue text.