Slack integration - Show Target branch in Merge events
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### Description
Right now when a merge event happens slack message only includes, user, branch name and event name. It would be really helpful to see target branch in the message so user's can tell the difference and would allow them to understand if it is an additional update or a different merge request to different branch. For instance
Serdar Sanri (Serdar.Sanri) opened 253 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds* in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds*
Serdar Sanri (Serdar.Sanri) merged 253 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds* in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds*
Serdar Sanri (Serdar.Sanri) opened [254 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*] in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*
Serdar Sanri (Serdar.Sanri) merged [254 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*] in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*
Serdar Sanri (Serdar.Sanri) opened [255 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*] in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*
Serdar Sanri (Serdar.Sanri) merged [255 *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*] in root/project-web: *Gitlabci update- disabled staging build causing artifact transfer to csapp builds [skip ci]*
When someone looks at the above messages in channel, thinks that I created 3 merge request and accepted them. First one was ran pipeline then other two skipped pipeline. But in reality this three MRs are made to three different upstream branches.
First one was to release branch to kick off a build and the other two was to master and qa branches. So as a developer I have no way to tell from slack messages.
### Proposal
Adding target branch name to actual message text would be really helpful
issue