Fix importing PRs with not existing branches

lib/bitbucket/connection.rb

@@ -45,7 +45,7 @@ module Bitbucket
       @expires_at    = response.expires_at
       @expires_in    = response.expires_in
       @refresh_token = response.refresh_token
-      @connection = nil
+      @connection    = nil
     end
 
     private

lib/gitlab/bitbucket_import/importer.rb

@@ -81,10 +81,10 @@ module Gitlab
               description: description,
               source_project: project,
               source_branch: pull_request.source_branch_name,
-              source_branch_sha: project.repository.rugged.lookup(pull_request.source_branch_sha).oid,
+              source_branch_sha: pull_request.source_branch_sha,
               target_project: project,
               target_branch: pull_request.target_branch_name,
-              target_branch_sha: project.repository.rugged.lookup(pull_request.target_branch_sha).oid,
+              target_branch_sha: pull_request.target_branch_sha,
               state: pull_request.state,
               author_id: gitlab_user_id(project, pull_request.author),
               assignee_id: nil,