Just trust set_commit_variables to set everything!

app/controllers/concerns/creates_commit.rb

@@ -4,11 +4,9 @@ module CreatesCommit
   def create_commit(service, success_path:, failure_path:, failure_view: nil, success_notice: nil)
     set_commit_variables
 
-    source_branch = @ref if
-      @ref && @mr_target_project.repository.branch_exists?(@ref)
     commit_params = @commit_params.merge(
       source_project: @mr_target_project,
-      source_branch: source_branch,
+      source_branch: @mr_target_branch,
       target_branch: @mr_source_branch
     )
 

app/controllers/projects/commit_controller.rb

@@ -40,7 +40,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def revert
-    assign_change_commit_vars(@commit.revert_branch_name)
+    assign_change_commit_vars
 
     return render_404 if @target_branch.blank?
 
@@ -49,7 +49,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def cherry_pick
-    assign_change_commit_vars(@commit.cherry_pick_branch_name)
+    assign_change_commit_vars
 
     return render_404 if @target_branch.blank?
 
@@ -110,11 +110,9 @@ class Projects::CommitController < Projects::ApplicationController
     @ci_pipelines = project.pipelines.where(sha: commit.sha)
   end
 
-  def assign_change_commit_vars(mr_source_branch)
+  def assign_change_commit_vars
     @commit = project.commit(params[:id])
     @target_branch = params[:target_branch]
-    @mr_source_branch = mr_source_branch
-    @mr_target_branch = @target_branch
     @commit_params = {
       commit: @commit,
       create_merge_request: params[:create_merge_request].present? || different_project?

app/services/commits/change_service.rb

@@ -5,6 +5,7 @@ module Commits
 
     def execute
       @source_project = params[:source_project] || @project
+      @source_branch = params[:source_branch]
       @target_branch = params[:target_branch]
       @commit = params[:commit]
       @create_merge_request = params[:create_merge_request].present?
@@ -38,7 +39,7 @@ module Commits
           into,
           tree_id,
           source_project: @source_project,
-          source_branch_name: @target_branch)
+          source_branch_name: @source_branch)
 
         success
       else