Include additional information in merge request email notification
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#### Overview
A customer is trying to apply email filtering based on merge request email notifications. They're requesting to filter by the following attributes:
* The target branch for the merge request
* Something in the email that says it's a merge request
Currently the [merge request note notification](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/notify/note_merge_request_email.html.haml) does not contain this information.
#### Proposal
Update the email note MR template to include the target branch and the words "Merge request"
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ZD: https://gitlab.zendesk.com/agent/tickets/28115
issue