SSRF in restore-repo via unsanitized pull_request.yml Head.CloneURL

Affected: Go: code.gitea.io/gitea

### Summary Gitea's `restore-repo` CLI command restores a repository from a dump directory/archive. When parsing `pull_request.yml` from that dump, the `Head.CloneURL` field is used to add a git remote and fetch from it with no validation, because the safety check that's supposed to guard it (`CheckAndEnsureSafePR`) is called with an empty `commonCloneBaseURL`, which silently disables it. This lets a malicious dump make the Gitea server execute `git fetch` against an attacker-chosen URL (SSRF), or disclose a local git repository via `file://`. This is a different root cause from the recently fixed path-traversal issue in the same command (#38215), which patched `DownloadURL`/`PatchURL` but not `Head.CloneURL`.

### Details `services/migrations/restore.go`'s `GetPullRequests()` unmarshals `pull_request.yml` directly into `base.PullRequest` structs with no validation of `Head.CloneURL`:

```go err = yaml.Unmarshal(bs, &pulls) ... for _, pr := range pulls { if pr.PatchURL != "" { pr.PatchURL = "file://" + util.FilePathJoinAbs(r.baseDir, pr.PatchURL) } CheckAndEnsureSafePR(pr, "", r) // <-- empty baseURL } ```

`CheckAndEnsureSafePR` (`services/migrations/common.go`) is supposed to reject `Head.CloneURL`/`PatchURL` values that don't share a common base URL:

```go func hasBaseURL(toCheck, baseURL string) bool { if len(baseURL) > 0 && baseURL[len(baseURL)-1] != '/' { baseURL += "/" } return strings.HasPrefix(toCheck, baseURL) }

func Chec…

Ein Angreifer kann mehrere Schwachstellen in Gitea ausnutzen, um beliebigen Programmcode auszuführen, erweiterte Berechtigungen zu erlangen, Sicherheitsmaßnahmen zu umgehen, Daten zu manipulieren, sensible Informationen offenzulegen, Sitzungen zu übernehmen oder wiederzuverwenden oder einen Denial-of-Service-Zustand auszulösen.