Load PR Issue Poster on API too (#11033) (#11039)

* Load pr Issue Poster on API too (#11033)

* ajust for 1.11 codebase

Author: 6543

services/pull/merge.go

@@ -285,6 +285,10 @@ func Merge(pr *models.PullRequest, doer *models.User, baseGitRepo *git.Repositor
 			return err
 		}
 
+		if err = pr.Issue.LoadPoster(); err != nil {
+			log.Error("LoadPoster: %v", err)
+			return fmt.Errorf("LoadPoster: %v", err)
+		}
 		sig := pr.Issue.Poster.NewGitSig()
 		if signArg == "" {
 			if err := git.NewCommand("commit", fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email), "-m", message).RunInDirTimeoutEnvPipeline(env, -1, tmpBasePath, &outbuf, &errbuf); err != nil {