Don't print file header when it's not present.

This is needed for rename FileDiffs that do not have a file header.

Author: dmitshur

diff/print.go

@@ -36,11 +36,13 @@ func PrintFileDiff(d *FileDiff) ([]byte, error) {
 		}
 	}
 
-	if err := printFileHeader(&buf, "--- ", d.OrigName, timePtr(d.OrigTime)); err != nil {
-		return nil, err
-	}
-	if err := printFileHeader(&buf, "+++ ", d.NewName, timePtr(d.NewTime)); err != nil {
-		return nil, err
+	if d.OrigName != "" || d.OrigTime != nil || d.NewName != "" || d.NewTime != nil {
+		if err := printFileHeader(&buf, "--- ", d.OrigName, timePtr(d.OrigTime)); err != nil {
+			return nil, err
+		}
+		if err := printFileHeader(&buf, "+++ ", d.NewName, timePtr(d.NewTime)); err != nil {
+			return nil, err
+		}
 	}
 
 	ph, err := PrintHunks(d.Hunks)