Use WriteRef to create/delete branches w/o hooks

The GitLab::Git::Repository class had two methods which aren't really
used in production. `#create_branch` and `#remove_branch` now leverage
WriteRef. The behaviour does change now, in the case of `#create_branch`
it doesn't throw an error anymore if the branch already exists. That's
actually not a problem as all call sites are import related, in which
case there's only one branch to be imported.

`#delete_branch` is only used in tests. The runtime errors aren't
depended on in that case. Again, this change in behaviour doesn't
influence GitLab-Rails.

lib/gitlab/git/repository.rb

@@ -18,6 +18,7 @@ module Gitlab
       GITALY_INTERNAL_URL = 'ssh://gitaly/internal.git'
       GITLAB_PROJECTS_TIMEOUT = Gitlab.config.gitlab_shell.git_timeout
       EMPTY_REPOSITORY_CHECKSUM = '0000000000000000000000000000000000000000'
+      REF_REMOVAL_UPDATE_REV = '0' * 40
 
       NoRepository = Class.new(StandardError)
       InvalidRepository = Class.new(StandardError)
@@ -636,10 +637,9 @@ module Gitlab
       end
 
       # Delete the specified branch from the repository
+      # Note: No Git hooks are executed for this action
       def delete_branch(branch_name)
-        wrapped_gitaly_errors do
-          gitaly_ref_client.delete_branch(branch_name)
-        end
+        write_ref(branch_name, REF_REMOVAL_UPDATE_REV)
       rescue CommandError => e
         raise DeleteBranchError, e
       end
@@ -651,14 +651,13 @@ module Gitlab
       end
 
       # Create a new branch named **ref+ based on **stat_point+, HEAD by default
+      # Note: No Git hooks are executed for this action
       #
       # Examples:
       #   create_branch("feature")
       #   create_branch("other-feature", "master")
       def create_branch(ref, start_point = "HEAD")
-        wrapped_gitaly_errors do
-          gitaly_ref_client.create_branch(ref, start_point)
-        end
+        write_ref(ref, start_point)
       end
 
       # If `mirror_refmap` is present the remote is set as mirror with that mapping

lib/gitlab/import_export/merge_request_parser.rb

@@ -48,6 +48,14 @@ module Gitlab
         @project.repository.create_branch(@merge_request.target_branch, @merge_request.target_branch_sha)
       end
 
+      def fetch_ref
+        target_ref = Gitlab::Git::BRANCH_REF_PREFIX + @merge_request.source_branch
+
+        unless @project.repository.fetch_source_branch!(@project.repository, @diff_head_sha, target_ref)
+          Rails.logger.warn("Import/Export warning: Failed to create #{target_ref} for MR: #{@merge_request.iid}") # rubocop:disable Gitlab/RailsLogger
+        end
+      end
+
       def branch_exists?(branch_name)
         @project.repository.raw.branch_exists?(branch_name)
       end

spec/lib/gitlab/git/repository_spec.rb

@@ -420,55 +420,6 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
   end
 
-  describe "#delete_branch" do
-    let(:repository) { mutable_repository }
-
-    after do
-      ensure_seeds
-    end
-
-    it "removes the branch from the repo" do
-      branch_name = "to-be-deleted-soon"
-
-      repository.create_branch(branch_name)
-      expect(repository_rugged.branches[branch_name]).not_to be_nil
-
-      repository.delete_branch(branch_name)
-      expect(repository_rugged.branches[branch_name]).to be_nil
-    end
-
-    context "when branch does not exist" do
-      it "raises a DeleteBranchError exception" do
-        expect { repository.delete_branch("this-branch-does-not-exist") }.to raise_error(Gitlab::Git::Repository::DeleteBranchError)
-      end
-    end
-  end
-
-  describe "#create_branch" do
-    let(:repository) { mutable_repository }
-
-    after do
-      ensure_seeds
-    end
-
-    it "creates a new branch" do
-      expect(repository.create_branch('new_branch', 'master')).not_to be_nil
-    end
-
-    it "creates a new branch with the right name" do
-      expect(repository.create_branch('another_branch', 'master').name).to eq('another_branch')
-    end
-
-    it "fails if we create an existing branch" do
-      repository.create_branch('duplicated_branch', 'master')
-      expect {repository.create_branch('duplicated_branch', 'master')}.to raise_error("Branch duplicated_branch already exists")
-    end
-
-    it "fails if we create a branch from a non existing ref" do
-      expect {repository.create_branch('branch_based_in_wrong_ref', 'master_2_the_revenge')}.to raise_error("Invalid reference master_2_the_revenge")
-    end
-  end
-
   describe '#delete_refs' do
     let(:repository) { mutable_repository }