Expose refspec and depth to runner

What does this MR do?

This MR exposes refspecs and depth (for git-fetch) to runners according to this specification.

GitLab-Runner uses the exposed spec and depth in order to checkout source code.

We're going to use merge request's refs (e.g. refs/merge-requests/:iid/head) in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25504.

Sample response on POST jobs/request

When the job is not associated with a merge request

{
   "id":6594,
   "token":"FNWRozxyDZ4Q1HHWyW-1",
   "allow_git_fetch":true,
   "job_info":{
      "name":"deploy",
      "stage":"test",
      "project_id":89,
      "project_name":"prospective-merge-pipelines"
   },
   "git_info":{
      "repo_url":"http://gitlab-ci-token:FNWRozxyDZ4Q1HHWyW-1@localhost:8181/root/prospective-merge-pipelines.git",
      "sha":"abb6076c965eec51b92667388be8e7ef559f9ca3",
      "before_sha":"0000000000000000000000000000000000000000",
      "ref":"patch-7",
      "ref_type":"branch",
      "refspecs":[
         "+refs/heads/*:refs/remotes/origin/*",
         "+refs/tags/*:refs/tags/*"
      ],
      "depth":0
   },
   ...
}

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

app/models/ci/build.rb

@@ -46,6 +46,7 @@ class Build < CommitStatus
     delegate :terminal_specification, to: :runner_session, allow_nil: true
     delegate :gitlab_deploy_token, to: :project
     delegate :trigger_short_token, to: :trigger_request, allow_nil: true
+    delegate :merge_request?, to: :pipeline
 
     ##
     # Since Gitlab 11.5, deployments records started being created right after
@@ -441,11 +442,13 @@ def simple_variables
     # All variables, including persisted environment variables.
     #
     def variables
-      Gitlab::Ci::Variables::Collection.new
-        .concat(persisted_variables)
-        .concat(scoped_variables)
-        .concat(persisted_environment_variables)
-        .to_runner_variables
+      strong_memoize(:variables) do
+        Gitlab::Ci::Variables::Collection.new
+          .concat(persisted_variables)
+          .concat(scoped_variables)
+          .concat(persisted_environment_variables)
+          .to_runner_variables
+      end
     end
 
     ##
@@ -463,6 +466,8 @@ def features
     def merge_request
       return @merge_request if defined?(@merge_request)
 
+      @merge_request ||= pipeline.merge_request if merge_request?
+
       @merge_request ||=
         begin
           merge_requests = MergeRequest.includes(:latest_merge_request_diff)