Add support for create_tree_error_code from Gitaly

Available in Gitaly 1.70, this additional field on `CreateTreeError`
provides context about why a cherry-pick or revert failed.

This adds an `error_code` attribute to
`Gitlab::Git::Repository::CreateTreeError`.

app/services/commits/change_service.rb

@@ -23,7 +23,7 @@ module Commits
         message,
         start_project: @start_project,
         start_branch_name: @start_branch)
-    rescue Gitlab::Git::Repository::CreateTreeError
+    rescue Gitlab::Git::Repository::CreateTreeError => ex
       act = action.to_s.dasherize
       type = @commit.change_type_title(current_user)
 

lib/gitlab/git/repository.rb

@@ -25,9 +25,17 @@ module Gitlab
       InvalidRef = Class.new(StandardError)
       GitError = Class.new(StandardError)
       DeleteBranchError = Class.new(StandardError)
-      CreateTreeError = Class.new(StandardError)
       TagExistsError = Class.new(StandardError)
       ChecksumError = Class.new(StandardError)
+      class CreateTreeError < StandardError
+        attr_reader :error_code
+
+        def initialize(error_code)
+          super(self.class.name)
+
+          @error_code = error_code.downcase.to_sym
+        end
+      end
 
       # Directory name of repo
       attr_reader :name

lib/gitlab/gitaly_client/operation_service.rb

@@ -447,7 +447,7 @@ module Gitlab
         elsif response.commit_error.presence
           raise Gitlab::Git::CommitError, response.commit_error
         elsif response.create_tree_error.presence
-          raise Gitlab::Git::Repository::CreateTreeError, response.create_tree_error
+          raise Gitlab::Git::Repository::CreateTreeError, response.create_tree_error_code
         end
 
         Gitlab::Git::OperationService::BranchUpdate.from_gitaly(response.branch_update)