built-in add -i: support ? (prompt help)

With this change, we print out the same colored help text that the
Perl-based `git add -i` prints in the main loop when question mark is
entered.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

add-interactive.c

@@ -11,6 +11,7 @@ struct add_i_state {
 	struct repository *r;
 	int use_color;
 	char header_color[COLOR_MAXLEN];
+	char help_color[COLOR_MAXLEN];
 };
 
 static void init_color(struct repository *r, struct add_i_state *s,
@@ -43,6 +44,7 @@ static int init_add_i_state(struct repository *r, struct add_i_state *s)
 	s->use_color = want_color(s->use_color);
 
 	init_color(r, s, "header", s->header_color, GIT_COLOR_BOLD);
+	init_color(r, s, "help", s->help_color, GIT_COLOR_BOLD_RED);
 
 	return 0;
 }
@@ -104,6 +106,7 @@ struct list_and_choose_options {
 	struct list_options list_opts;
 
 	const char *prompt;
+	void (*print_help)(struct add_i_state *s);
 };
 
 /*
@@ -138,6 +141,11 @@ static ssize_t list_and_choose(struct prefix_item **items, size_t nr,
 		if (!input.len)
 			break;
 
+		if (!strcmp(input.buf, "?")) {
+			opts->print_help(s);
+			continue;
+		}
+
 		p = input.buf;
 		for (;;) {
 			size_t sep = strcspn(p, " \t\r\n,");
@@ -424,12 +432,24 @@ struct command_item {
 		       struct file_list *files, struct list_options *opts);
 };
 
+static void command_prompt_help(struct add_i_state *s)
+{
+	const char *help_color = s->help_color;
+	color_fprintf_ln(stdout, help_color, "%s", _("Prompt help:"));
+	color_fprintf_ln(stdout, help_color, "1          - %s",
+			 _("select a numbered item"));
+	color_fprintf_ln(stdout, help_color, "foo        - %s",
+			 _("select item based on unique prefix"));
+	color_fprintf_ln(stdout, help_color, "           - %s",
+			 _("(empty) select nothing"));
+}
+
 int run_add_i(struct repository *r, const struct pathspec *ps)
 {
 	struct add_i_state s = { NULL };
 	struct list_and_choose_options main_loop_opts = {
 		{ 4, N_("*** Commands ***"), print_command_item, NULL },
-		N_("What now")
+		N_("What now"), command_prompt_help
 	};
 	struct command_item
 		status = { { "status" }, run_status };