Slack Notifications are not working on First Failure Only when Git is used

### Your checklist for this issue

- [x] Jenkins version 2.190.1
- [x] Plugin version: Slack Notifications
- [ ] OS

### Description
I have 2 identical jobs that use **Slack Notifications**. They are both set to `Notify First Failure Only`.
However, Job 1 is set to use `Git` as the **Source Code Management**.
Job 1 sends a Slack notification with every build, while Job 2 sends the notifications on first failure only.
It doesn't make sense to me, but seems that setting the **Source Code Management** affects the settings of the **Slack Notifications**.

Reproduce the report with two otherwise identical Jenkins jobs, comparing Git SCM with the other configuration while both use “Notify First Failure Only.” Start by tracing the Slack notification logic that decides whether a build is the first failure; done means Git SCM no longer causes notifications on every build, with a regression test covering both job configurations.

Written by the indexing model from the issue text.