Generate notifications for Watchers when pushing to an MR's branch
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## Context
When:
* You watch a repo in gitlab.
* Someone creates an MR in the repo.
* Sometime pushes modified commits to the branch of the MR.
What happens is:
* No email notification is generated.
What's desired:
* Email notification is generated.
## Proposal
Send notifications for Watchers for push events to the MR source branch.
## Workaround
Switch from "Watching" to "Custom" notification setting for the repo.
issue