Remove extra space when writing first hunk when outside of context (#4075)

Author: paul-sachs

private/pkg/diff/diffmyers/diffmyers.go

@@ -144,8 +144,10 @@ func Print(from, to [][]byte, edits []Edit) ([]byte, error) {
 	var buffer bytes.Buffer
 	buffer.Grow(bufferSize)
 	for _, line := range out {
-		if line.hunk && len(line.line) > 0 {
-			buffer.Write(line.line)
+		if line.hunk {
+			if len(line.line) > 0 {
+				buffer.Write(line.line)
+			}
 			continue
 		}
 		switch line.EditKind {

private/pkg/diff/diffmyers/diffmyers_test.go

@@ -201,6 +201,32 @@ The door of all subtleties!
 		)
 		testPrint(t, lao, tzu, edits, "lao-tzu")
 	})
+
+	t.Run("first-line-prefix", func(t *testing.T) {
+		t.Parallel()
+		from := "syntax = \"proto3\";\n\npackage test;\n\nmessage Foo {\n  string field1 = 1;\n  string field2 = 2;\n  string field3 = 3;\n  string field4 = 4;\n  string field5 = 5;\n}\n"
+		to := "syntax = \"proto3\";\n\npackage test;\n\nmessage Foo {\n  string field1 = 1;\n  string field2 = 2;\n  string field3 = 3;\n  string field4 = 4;\n  int32 field5 = 5;\n}\n"
+		expectedFirstLineOfOutput := " syntax = \"proto3\";"
+		fromLines := splitLines(from)
+		toLines := splitLines(to)
+		edits := diffmyers.Diff(
+			fromLines,
+			toLines,
+		)
+		diff, err := diffmyers.Print(
+			fromLines,
+			toLines,
+			edits,
+		)
+		require.NoError(t, err)
+		firstLineEnd := bytes.Index(diff, []byte("\n"))
+
+		firstLine := diff[:firstLineEnd]
+		actualFirstLine := string(firstLine)
+		require.Equal(t, expectedFirstLineOfOutput, actualFirstLine,
+			"First line of diff output should match expected format (single space prefix, no double space)")
+		testPrint(t, from, to, edits, "first-line-prefix")
+	})
 }
 
 func testPrint(t *testing.T, from, to string, edits []diffmyers.Edit, golden string) {

private/pkg/diff/diffmyers/testdata/first-line-prefix

@@ -0,0 +1,13 @@
+ syntax = "proto3";
+ 
+ package test;
+ 
+ message Foo {
+   string field1 = 1;
+   string field2 = 2;
+   string field3 = 3;
+   string field4 = 4;
+@@ -10,1 +10,1 @@
+-  string field5 = 5;
++  int32 field5 = 5;
+ }