Don't execute git hooks if you create branch as part of other change

What does this MR do?

Currently, our procedure for adding a commit requires us to execute CreateBranchService before file creation. It's OK, but also we do execute git hooks (the PostReceive sidekiq job) as part of this process. However, this hook is execute before the file is actually committed, so the ref is updated.

Secondly, we do execute a git hooks after committing file and updating ref. This results in duplicate PostReceive jobs, where the first one is completely invalid.

This change makes the branch creation, something that is intermediate step of bigger process (file creation or update, commit cherry pick or revert) to not execute git hooks.

Why was this MR needed?

Adds a to CreateBranchService an option to disable hooks execution.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23439

app/controllers/concerns/creates_commit.rb

@@ -4,13 +4,15 @@ module CreatesCommit
   def create_commit(service, success_path:, failure_path:, failure_view: nil, success_notice: nil)
     set_commit_variables
 
+    start_branch = @mr_target_branch unless initial_commit?
     commit_params = @commit_params.merge(
-      source_project: @project,
-      source_branch: @ref,
-      target_branch: @target_branch
+      start_project: @mr_target_project,
+      start_branch: start_branch,
+      target_branch: @mr_source_branch
     )
 
-    result = service.new(@tree_edit_project, current_user, commit_params).execute
+    result = service.new(
+      @mr_source_project, current_user, commit_params).execute
 
     if result[:status] == :success
       update_flash_notice(success_notice)
@@ -89,20 +91,18 @@ module CreatesCommit
     @mr_source_project != @mr_target_project
   end
 
-  def different_branch?
-    @mr_source_branch != @mr_target_branch || different_project?
-  end
-
   def create_merge_request?
-    params[:create_merge_request].present? && different_branch?
+    # XXX: Even if the field is set, if we're checking the same branch
+    # as the target branch in the same project,
+    # we don't want to create a merge request.
+    params[:create_merge_request].present? &&
+      (different_project? || @ref != @target_branch)
   end
 
+  # TODO: We should really clean this up
   def set_commit_variables
-    @mr_source_branch ||= @target_branch
-
     if can?(current_user, :push_code, @project)
       # Edit file in this project
-      @tree_edit_project = @project
       @mr_source_project = @project
 
       if @project.forked?
@@ -112,15 +112,34 @@ module CreatesCommit
       else
         # Merge request to this project
         @mr_target_project = @project
-        @mr_target_branch ||= @ref
+        @mr_target_branch = @ref || @target_branch
       end
     else
-      # Edit file in fork
-      @tree_edit_project = current_user.fork_of(@project)
       # Merge request from fork to this project
-      @mr_source_project = @tree_edit_project
+      @mr_source_project = current_user.fork_of(@project)
       @mr_target_project = @project
-      @mr_target_branch ||= @ref
+      @mr_target_branch = @ref || @target_branch
     end
+
+    @mr_source_branch = guess_mr_source_branch
+  end
+
+  def initial_commit?
+    @mr_target_branch.nil? ||
+      !@mr_target_project.repository.branch_exists?(@mr_target_branch)
+  end
+
+  def guess_mr_source_branch
+    # XXX: Happens when viewing a commit without a branch. In this case,
+    # @target_branch would be the default branch for @mr_source_project,
+    # however we want a generated new branch here. Thus we can't use
+    # @target_branch, but should pass nil to indicate that we want a new
+    # branch instead of @target_branch.
+    return if
+      create_merge_request? &&
+          # XXX: Don't understand why rubocop prefers this indention
+          @mr_source_project.repository.branch_exists?(@target_branch)
+
+    @target_branch
   end
 end