emit_line(): don't emit an empty <SET><RESET> followed by a newline

When emit_line() is called with an empty line (but non-zero length, as we
send line terminating LF or CRLF to the function), it used to emit
<SET><RESET> followed by a newline.  Stop the wastefulness.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -295,12 +295,13 @@ static void emit_line_0(FILE *file, const char *set, const char *reset,
 		nofirst = 0;
 	}
 
-	fputs(set, file);
-
-	if (!nofirst)
-		fputc(first, file);
-	fwrite(line, len, 1, file);
-	fputs(reset, file);
+	if (len || !nofirst) {
+		fputs(set, file);
+		if (!nofirst)
+			fputc(first, file);
+		fwrite(line, len, 1, file);
+		fputs(reset, file);
+	}
 	if (has_trailing_carriage_return)
 		fputc('\r', file);
 	if (has_trailing_newline)

t/t4034-diff-words.sh

@@ -49,7 +49,7 @@ cat > expect <<\EOF
 <WHITE>+++ b/post<RESET>
 <BROWN>@@ -1,3 +1,7 @@<RESET>
 <RED>h(4)<RESET><GREEN>h(4),hh[44]<RESET>
-<RESET>
+
 a = b + c<RESET>
 
 <GREEN>aa = a<RESET>
@@ -90,7 +90,7 @@ cat > expect <<\EOF
 <WHITE>+++ b/post<RESET>
 <BROWN>@@ -1,3 +1,7 @@<RESET>
 h(4),<GREEN>hh<RESET>[44]
-<RESET>
+
 a = b + c<RESET>
 
 <GREEN>aa = a<RESET>
@@ -126,7 +126,7 @@ cat > expect <<\EOF
 <WHITE>+++ b/post<RESET>
 <BROWN>@@ -1,3 +1,7 @@<RESET>
 h(4)<GREEN>,hh[44]<RESET>
-<RESET>
+
 a = b + c<RESET>
 
 <GREEN>aa = a<RESET>
@@ -168,7 +168,7 @@ cat > expect <<\EOF
 <WHITE>+++ b/post<RESET>
 <BROWN>@@ -1,3 +1,7 @@<RESET>
 h(4),<GREEN>hh[44<RESET>]
-<RESET>
+
 a = b + c<RESET>
 
 <GREEN>aa = a<RESET>