diff.c: add set_sign to emit_line_0

Split the meaning of the `set` parameter that is passed to
emit_line_0()` to separate between the color of the "sign" (i.e.
the diff marker '+', '-' or ' ' that is passed in as the `first`
parameter) and the color of the rest of the line.

This changes the meaning of the `set` parameter to no longer refer
to the color of the diff marker, but instead to refer to the color
of the rest of the line. A value of `NULL` indicates that the rest
of the line wants to be colored the same as the diff marker.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -622,7 +622,7 @@ static void check_blank_at_eof(mmfile_t *mf1, mmfile_t *mf2,
 }
 
 static void emit_line_0(struct diff_options *o,
-			const char *set, unsigned reverse, const char *reset,
+			const char *set_sign, const char *set, unsigned reverse, const char *reset,
 			int first, const char *line, int len)
 {
 	int has_trailing_newline, has_trailing_carriage_return;
@@ -652,9 +652,15 @@ static void emit_line_0(struct diff_options *o,
 	if (len || !nofirst) {
 		if (reverse && want_color(o->use_color))
 			fputs(GIT_COLOR_REVERSE, file);
-		fputs(set, file);
+		if (set_sign)
+			fputs(set_sign, file);
 		if (first && !nofirst)
 			fputc(first, file);
+		if (set && set != set_sign) {
+			if (set_sign)
+				fputs(reset, file);
+			fputs(set, file);
+		}
 		fwrite(line, len, 1, file);
 		fputs(reset, file);
 	}
@@ -667,7 +673,7 @@ static void emit_line_0(struct diff_options *o,
 static void emit_line(struct diff_options *o, const char *set, const char *reset,
 		      const char *line, int len)
 {
-	emit_line_0(o, set, 0, reset, line[0], line+1, len-1);
+	emit_line_0(o, set, NULL, 0, reset, line[0], line+1, len-1);
 }
 
 enum diff_symbol {
@@ -1199,17 +1205,17 @@ static void emit_line_ws_markup(struct diff_options *o,
 	}
 
 	if (!ws && !set_sign)
-		emit_line_0(o, set, 0, reset, sign, line, len);
+		emit_line_0(o, set, NULL, 0, reset, sign, line, len);
 	else if (!ws) {
 		/* Emit just the prefix, then the rest. */
-		emit_line_0(o, set_sign, !!set_sign, reset, sign, "", 0);
-		emit_line_0(o, set, 0, reset, 0, line, len);
+		emit_line_0(o, set_sign, NULL, !!set_sign, reset, sign, "", 0);
+		emit_line_0(o, set, NULL, 0, reset, 0, line, len);
 	} else if (blank_at_eof)
 		/* Blank line at EOF - paint '+' as well */
-		emit_line_0(o, ws, 0, reset, sign, line, len);
+		emit_line_0(o, ws, NULL, 0, reset, sign, line, len);
 	else {
 		/* Emit just the prefix, then the rest. */
-		emit_line_0(o, set_sign ? set_sign : set, !!set_sign, reset,
+		emit_line_0(o, set_sign ? set_sign : set, NULL, !!set_sign, reset,
 			    sign, "", 0);
 		ws_check_emit(line, len, ws_rule,
 			      o->file, set, reset, ws);
@@ -1233,7 +1239,7 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 		context = diff_get_color_opt(o, DIFF_CONTEXT);
 		reset = diff_get_color_opt(o, DIFF_RESET);
 		putc('\n', o->file);
-		emit_line_0(o, context, 0, reset, '\\',
+		emit_line_0(o, context, NULL, 0, reset, '\\',
 			    nneof, strlen(nneof));
 		break;
 	case DIFF_SYMBOL_SUBMODULE_HEADER: