Handle push rejection message in Merge & Web Editor (#10373)

* Handle push rejection message in Merge

* placate golangci-lint

* Fix sanitize, adjust message handling

* oops

* Oops

* Handle push-rejection in webeditor CRUD too

* Apply suggestions from code review

Co-authored-by: Lauris BH <lauris@nix.lv>

services/pull/merge.go

@@ -392,6 +392,15 @@ func rawMerge(pr *models.PullRequest, doer *models.User, mergeStyle models.Merge
 				StdErr: errbuf.String(),
 				Err:    err,
 			}
+		} else if strings.Contains(errbuf.String(), "! [remote rejected]") {
+			err := models.ErrPushRejected{
+				Style:  mergeStyle,
+				StdOut: outbuf.String(),
+				StdErr: errbuf.String(),
+				Err:    err,
+			}
+			err.GenerateMessage()
+			return "", err
 		}
 		return "", fmt.Errorf("git push: %s", errbuf.String())
 	}