Skip file headers if FromFile/ToFile are empty

difflib/difflib.go

diff --git a/difflib/difflib.go b/difflib/difflib.go
index e5005da..003e99f 100644
--- a/difflib/difflib.go
+++ b/difflib/difflib.go
@@ -585,13 +585,15 @@
 			if len(diff.ToDate) > 0 {
 				toDate = "\t" + diff.ToDate
 			}
-			err := wf("--- %s%s%s", diff.FromFile, fromDate, diff.Eol)
-			if err != nil {
-				return err
-			}
-			err = wf("+++ %s%s%s", diff.ToFile, toDate, diff.Eol)
-			if err != nil {
-				return err
+			if diff.FromFile != "" || diff.ToFile != "" {
+				err := wf("--- %s%s%s", diff.FromFile, fromDate, diff.Eol)
+				if err != nil {
+					return err
+				}
+				err = wf("+++ %s%s%s", diff.ToFile, toDate, diff.Eol)
+				if err != nil {
+					return err
+				}
 			}
 		}
 		first, last := g[0], g[len(g)-1]
@@ -710,8 +712,10 @@
 			if len(diff.ToDate) > 0 {
 				toDate = "\t" + diff.ToDate
 			}
-			wf("*** %s%s%s", diff.FromFile, fromDate, diff.Eol)
-			wf("--- %s%s%s", diff.ToFile, toDate, diff.Eol)
+			if diff.FromFile != "" || diff.ToFile != "" {
+				wf("*** %s%s%s", diff.FromFile, fromDate, diff.Eol)
+				wf("--- %s%s%s", diff.ToFile, toDate, diff.Eol)
+			}
 		}
 
 		first, last := g[0], g[len(g)-1]