diff.c: add --output-indicator-{new, old, context}

This will prove useful in range-diff in a later patch as we will be able to
differentiate between adding a new file (that line is starting with +++
and then the file name) and regular new lines.

It could also be useful for experimentation in new patch formats, i.e.
we could teach git to emit moved lines with lines other than +/-.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1281,7 +1281,9 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 			else if (c == '-')
 				set = diff_get_color_opt(o, DIFF_FILE_OLD);
 		}
-		emit_line_ws_markup(o, set_sign, set, reset, ' ', line, len,
+		emit_line_ws_markup(o, set_sign, set, reset,
+				    o->output_indicators[OUTPUT_INDICATOR_CONTEXT],
+				    line, len,
 				    flags & (DIFF_SYMBOL_CONTENT_WS_MASK), 0);
 		break;
 	case DIFF_SYMBOL_PLUS:
@@ -1324,7 +1326,9 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 				set = diff_get_color_opt(o, DIFF_CONTEXT_BOLD);
 			flags &= ~DIFF_SYMBOL_CONTENT_WS_MASK;
 		}
-		emit_line_ws_markup(o, set_sign, set, reset, '+', line, len,
+		emit_line_ws_markup(o, set_sign, set, reset,
+				    o->output_indicators[OUTPUT_INDICATOR_NEW],
+				    line, len,
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK,
 				    flags & DIFF_SYMBOL_CONTENT_BLANK_LINE_EOF);
 		break;
@@ -1367,7 +1371,9 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 			else
 				set = diff_get_color_opt(o, DIFF_CONTEXT_DIM);
 		}
-		emit_line_ws_markup(o, set_sign, set, reset, '-', line, len,
+		emit_line_ws_markup(o, set_sign, set, reset,
+				    o->output_indicators[OUTPUT_INDICATOR_OLD],
+				    line, len,
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK, 0);
 		break;
 	case DIFF_SYMBOL_WORDS_PORCELAIN:
@@ -4382,6 +4388,9 @@ void diff_setup(struct diff_options *options)
 
 	options->file = stdout;
 
+	options->output_indicators[OUTPUT_INDICATOR_NEW] = '+';
+	options->output_indicators[OUTPUT_INDICATOR_OLD] = '-';
+	options->output_indicators[OUTPUT_INDICATOR_CONTEXT] = ' ';
 	options->abbrev = DEFAULT_ABBREV;
 	options->line_termination = '\n';
 	options->break_opt = -1;
@@ -4869,6 +4878,12 @@ int diff_opt_parse(struct diff_options *options,
 		 options->output_format |= DIFF_FORMAT_DIFFSTAT;
 	} else if (!strcmp(arg, "--no-compact-summary"))
 		 options->flags.stat_with_summary = 0;
+	else if (skip_prefix(arg, "--output-indicator-new=", &arg))
+		options->output_indicators[OUTPUT_INDICATOR_NEW] = arg[0];
+	else if (skip_prefix(arg, "--output-indicator-old=", &arg))
+		options->output_indicators[OUTPUT_INDICATOR_OLD] = arg[0];
+	else if (skip_prefix(arg, "--output-indicator-context=", &arg))
+		options->output_indicators[OUTPUT_INDICATOR_CONTEXT] = arg[0];
 
 	/* renames options */
 	else if (starts_with(arg, "-B") ||

diff.h

@@ -194,6 +194,11 @@ struct diff_options {
 	FILE *file;
 	int close_file;
 
+#define OUTPUT_INDICATOR_NEW 0
+#define OUTPUT_INDICATOR_OLD 1
+#define OUTPUT_INDICATOR_CONTEXT 2
+	char output_indicators[3];
+
 	struct pathspec pathspec;
 	pathchange_fn_t pathchange;
 	change_fn_t change;