Use a migration test instead of a wrong test which populated the meta test repositories and fix a migration bug (#36160)

The test `TestGiteaUploadUpdateGitForPullRequest` modified the shared
meta test repositories directly, so this PR removes that test and
replaces it with an integration test that migrates a real repository
from gitea.com into a local test instance.

This PR also fixes a bug where pull-request migrations were not
correctly syncing head branches to the database.

services/migrations/dump.go

@@ -358,6 +358,11 @@ func (g *RepositoryDumper) SyncTags(ctx context.Context) error {
 	return nil
 }
 
+// SyncBranches syncs branches in the database
+func (g *RepositoryDumper) SyncBranches(ctx context.Context) error {
+	return nil
+}
+
 // CreateIssues creates issues
 func (g *RepositoryDumper) CreateIssues(_ context.Context, issues ...*base.Issue) error {
 	var err error