patchutil: handle nil-able combined patches

this was a known issue, but just ignored until now. when a patch reverts
all changes from an older patch, the combined patch for that file is
nil.

patchutil/combinediff.go

···
 				fmt.Println(err)
 			}
 
-			result = append(result, combined)
+			// combined can be nil commit 2 reverted all changes from commit 1
+			if combined != nil {
+				result = append(result, combined)
+			}
+
 		} else {
 			// only in patch1; add as-is
 			result = append(result, f1)