Support merge request create with push options

To create a new merge request:

  git push -u origin -o merge_request.create

To create a new merge request setting target branch:

  git push -u origin -o merge_request.create \
    -o merge_request.target=123

To update an existing merge request with a new target branch:

  git push -u origin -o merge_request.target=123

A new Gitlab::PushOptions class handles parsing and validating the push
options array. This can be the start of the standard of GitLab accepting
push options that follow namespacing rules. Rules are discussed in issue
https://gitlab.com/gitlab-org/gitlab-ce/issues/43263.

E.g. these push options:

  -o merge_request.create -o merge_request.target=123

Become parsed as:

  {
    merge_request: {
      create: true,
      target: '123',
    }
  }

And are fetched with the class via:

  push_options.get(:merge_request)
  push_options.get(:merge_request, :create)
  push_options.get(:merge_request, :target)

A new MergeRequests::PushOptionsHandlerService takes the `merge_request`
namespaced push options and handles creating and updating
merge requests.

Any errors encountered are passed to the existing `output` Hash in
Api::Internal's `post_receive` endpoint, and passed to gitlab-shell
where they're output to the user.

Issue https://gitlab.com/gitlab-org/gitlab-ce/issues/43263

1 files changed, 33 insertions, 0 deletions

diff --git a/doc/user/project/merge_requests/index.md b/doc/user/project/merge_requests/index.md
index 7c0380152de..678fc3dd196 100644
--- a/doc/user/project/merge_requests/index.md
+++ b/doc/user/project/merge_requests/index.md
@@ -219,6 +219,39 @@ apply the patches. The target branch can be specified using the
 [`/target_branch` quick action](../quick_actions.md). If the source
 branch already exists, the patches will be applied on top of it.
 
+## Git push options
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26752) in GitLab 11.10.
+
+GitLab supports using [Git push options](https://git-scm.com/docs/git-push#Documentation/git-push.txt--oltoptiongt) to create merge requests and set the target
+branch during a push. Note that git push options are only available with
+Git 2.10 or newer.
+
+### Create a new merge request using git push options
+
+To create a new merge request for a branch, use the
+`merge_request.create` push option:
+
+```sh
+git push -o merge_request.create
+```
+
+### Set the target branch of a merge request using git push options
+
+To update an existing merge request's target branch, use the
+`merge_request.target=<branch_name>` push option:
+
+```sh
+git push -o merge_request.target=branch_name
+```
+
+You can also create a merge request and set its target branch at the
+same time using a `-o` flag per push option:
+
+```sh
+git push -o merge_request.create -o merge_request.target=branch_name
+```
+
 ## Find the merge request that introduced a change
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/2383) in GitLab 10.5.