Improve discoverability when pushing code to GitLab

Problem to solve

When pushing code to a GitLab repository, in special ocasions we provide a URL that either allow user to follow up on the web UI (like when pushing code to a branch that has a MR open), or allow them to trigger an action that can only be done on the Web UI (when pushing a new branch to the repository, a URL is sent that allows the user to create a Merge Request).

There are other situations where may be useful for the user to have a quick URL bringing them back to the Web UI. We should explore few alternatives here.

Further details

• @sytses said here: https://gitlab.com/gitlab-org/gitlab-ce/issues/53012#note_111182448

  when I push a commit to the default brach I would love to get a url back that shows the commit like gitlab-com/www-gitlab-com@c7ec37c7 so I can share that with co-workers.

• @stanhu said here: https://gitlab.com/gitlab-org/gitlab-ce/issues/53012#note_111183107

  I often want to change the target branch (e.g. for security releases), so it would be nice if there were a way to also supply this in a friendly way. Right now I append something like merge_request[target_branch]=my-target-branch to the URL.

Proposal

When pushing to the default branch (usually master), instead of displaying something like this:

remote: 
remote: View merge request for 1234-awesome-branch:
remote:   https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/1234
remote:

Display something like this:

remote: 
remote: You've pushed directly to 'master'. You can view your last commit in GitLab:
remote:   https://gitlab.com/gitlab-com/www-gitlab-com/commit/c7ec37c7e8db83009a6c3b754a0c108828326730
remote:

To do that we need to edit the internal API, make it send the last commit URL when pushing to the main branch. We currently have this internal endpoint: /internal/merge_request_urls we probably want to rename this to something that describes returning both a merge request or a commit url (like: /internal/gitlab_reference_url. We may also want to change returned params to indicate when it's a merge request or when it's a commit URL we are sending, so gitlab-shell and gitlab-workhorse can build the message accordingly.

---

For the second item, app/controllers/projects/merge_requests/creations_controller.rb should accept simple query url param: target_branch when building the merge request object which will build the new merge request screen.

What does success look like, and how can we measure that?

1. Being able to set a target branch by using a URL pattern like:

https://#{gitlab.host}/:namespace/:project_path/merge_requests/new/:source_branch_name?target_branch=:target_branch_name

2. When pushing to the default branch, receive a message like in the example below:

remote: 
remote: You've pushed directly to 'master'. You can view your last commit in GitLab:
remote:   https://gitlab.com/gitlab-com/www-gitlab-com/commit/c7ec37c7e8db83009a6c3b754a0c108828326730
remote:

Links / references