Missing instruction for a custom branch
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### Problem to solve
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Since we support [Custom initial branch name](https://docs.gitlab.com/ee/user/project/repository/branches/#custom-initial-branch-name). We do not have instruction to push to a custom branch in [empty.html.haml](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/projects/empty.html.haml)
### Further details
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Git client creates default branch as `master`. The customer has to change from `master` branch to the customer branch example `main` either by
```
git checkout -b main # creates the main branch from the command line, git client defaults to master
```
or by
```
git branch --move master main
```
### Proposal
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The proposal is to add another section to improve our instruction to help customers with custom branch push their code.
Example.
```
%fieldset
%h5= _('Push to a custom branch')
%pre.bg-light
:preserve
cd existing_folder
git init
git remote add origin #{ content_tag(:span, default_url_to_repo, class: 'js-clone')}
git add .
git commit -m "Initial commit"
git branch --move master #{ default_branch_name }
- if @project.can_current_user_push_to_default_branch?
%span><
git push -u origin #{ default_branch_name }
```
### Who can address the issue
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### Other links/references
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[Zendesk ticket](https://gitlab.zendesk.com/agent/tickets/189382) (internal only)
issue