Use ArgumentError error instead because it's a bug

if it happens. Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7237#note_19747933

app/services/git_operation_service.rb

@@ -151,17 +151,17 @@ class GitOperationService
       end
 
       unless source_project.repository.branch_exists?(source_branch_name)
-        raise Repository::CommitError.new(
+        raise ArgumentError,
           "Cannot find branch #{branch_name} nor" \
           " #{source_branch_name} from" \
-          " #{source_project.path_with_namespace}")
+          " #{source_project.path_with_namespace}"
       end
     elsif source_branch_name
       unless repository.branch_exists?(source_branch_name)
-        raise Repository::CommitError.new(
+        raise ArgumentError,
           "Cannot find branch #{branch_name} nor" \
           " #{source_branch_name} from" \
-          " #{repository.project.path_with_namespace}")
+          " #{repository.project.path_with_namespace}"
       end
     end
   end