Add migrate from OneDev (#16356)

* Use context to simplify logic.

* Added migration from OneDev.
This PR adds [OneDev](https://code.onedev.io/) as migration source.

Supported:
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modules/migrations/gitea_uploader.go

@@ -609,6 +609,9 @@ func (g *GiteaLocalUploader) newPullRequest(pr *base.PullRequest) (*models.PullR
 
 	// download patch file
 	err := func() error {
+		if pr.PatchURL == "" {
+			return nil
+		}
 		// pr.PatchURL maybe a local file
 		ret, err := uri.Open(pr.PatchURL)
 		if err != nil {