Update GitHub PRs Importer to force update repository

  - Set `forced: true` flag when fetching remote git
    repository during import from GitHub in order to not
    fail import if any of the refs (tags specifically)
    get changed mid import

Changelog: fixed

lib/gitlab/github_import/importer/pull_requests_importer.rb

@@ -40,7 +40,7 @@ module Gitlab
           # updating the timestamp.
           project.touch(:last_repository_updated_at) # rubocop: disable Rails/SkipsModelValidations
 
-          project.repository.fetch_remote(project.import_url, refmap: Gitlab::GithubImport.refmap, forced: false)
+          project.repository.fetch_remote(project.import_url, refmap: Gitlab::GithubImport.refmap, forced: true)
 
           pname = project.path_with_namespace
 

spec/lib/gitlab/github_import/importer/pull_requests_importer_spec.rb

@@ -164,7 +164,7 @@ RSpec.describe Gitlab::GithubImport::Importer::PullRequestsImporter do
 
       expect(project.repository)
         .to receive(:fetch_remote)
-        .with(url, forced: false, refmap: Gitlab::GithubImport.refmap)
+        .with(url, forced: true, refmap: Gitlab::GithubImport.refmap)
 
       freeze_time do
         importer.update_repository