Merge request terms are reversed for GitOperationService

This is seriously confusing but a target branch in merge request,
is actually the source branch for GitOperationService, which is
the base branch. The source branch in a merge request, is the
target branch for GitOperationService, which is where we want to
make commits.

Perhaps we should rename source branch in GitOperationService to
base branch, and target branch to committing branch.

app/controllers/concerns/creates_commit.rb

@@ -5,15 +5,15 @@ module CreatesCommit
     set_commit_variables
 
     source_branch = @ref if
-      @ref && @mr_source_project.repository.branch_exists?(@ref)
+      @ref && @mr_target_project.repository.branch_exists?(@ref)
     commit_params = @commit_params.merge(
-      source_project: @mr_source_project,
+      source_project: @mr_target_project,
       source_branch: source_branch,
-      target_branch: @mr_target_branch
+      target_branch: @mr_source_branch
     )
 
     result = service.new(
-      @tree_edit_project, current_user, commit_params).execute
+      @mr_source_project, current_user, commit_params).execute
 
     if result[:status] == :success
       update_flash_notice(success_notice)