Configure the maximum amount of commits sent to matrix

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**Is your feature request related to a problem? Please describe.**
> You limit the amount of commits to the last 5 in case of pushing more that that at once. I can see the reasoning behind that, you might not want to spam the room with loads of commits when you subscribe to pushes in order to be notified that there was a push.
> Personally my workflow using GitLab webhooks (with the old bridge and [some jinja](https://github.com/HarHarLinks/matrix-webhook-receiver/blob/main/examples/gitlab.jinja2)) has been to review all of the commit messages on matrix, because after all I want to be lazy and not have to click these links.

**Describe the solution you'd like**
Add setting to room state for GitHub and GitLab.

The payload names no files or tests; start by tracing the GitHub and GitLab push-handling entry points and their room-state configuration. Define how a configurable commit limit should work for both integrations, then verify the current default of five commits remains unchanged when no setting is supplied.

Written by the indexing model from the issue text.