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…