Doesn't import GitHub PR where branches were no longer available

lib/gitlab/github_import/importer.rb

@@ -52,19 +52,18 @@ module Gitlab
         client.pull_requests(project.import_source, state: :all,
                                                     sort: :created,
                                                     direction: :asc).each do |pull_request|
-          body = @formatter.author_line(pull_request.user.login)
-          body += pull_request.body || ""
-
-          source_branch = pull_request.head.ref
-          target_branch = pull_request.base.ref
+          source_branch = find_branch(pull_request.head.ref)
+          target_branch = find_branch(pull_request.base.ref)
 
+          if source_branch && target_branch
+            # Pull Request
             merge_request = MergeRequest.create!(
               title: pull_request.title,
-            description: body,
+              description: format_body(pull_request.user.login, pull_request.body),
               source_project: project,
-            source_branch: source_branch,
+              source_branch: source_branch.name,
               target_project: project,
-            target_branch: target_branch,
+              target_branch: target_branch.name,
               state: merge_request_state(pull_request),
               author_id: gl_author_id(project, pull_request.user.id),
               assignee_id: gl_user_id(pull_request.assignee.try(:id)),
@@ -72,6 +71,7 @@ module Gitlab
               updated_at: pull_request.updated_at
             )
 
+            # Comments on Pull Request
             client.issue_comments(project.import_source, pull_request.number).each do |c|
               merge_request.notes.create!(
                 project: project,
@@ -82,6 +82,7 @@ module Gitlab
               )
             end
 
+            # Comments on Pull Request diff
             client.pull_request_comments(project.import_source, pull_request.number).each do |c|
               merge_request.notes.create!(
                 project: project,
@@ -95,6 +96,11 @@ module Gitlab
             end
           end
         end
+      end
+
+      def find_branch(name)
+        project.repository.find_branch(name)
+      end
 
       def format_body(author, body)
         @formatter.author_line(author) + (body || "")