sam: use default shell command for <, >, | and ! when applicaple

If the shell command is omitted, the last shell command (of any type)
is substituted. The most recently used shell command is stored in a
new register currently named `!`.

main.c

@@ -1265,6 +1265,8 @@ static const char *key2register(Vis *vis, const char *keys, enum VisRegister *re
 		*reg = VIS_REG_SEARCH;
 	else if (keys[0] == ':')
 		*reg = VIS_REG_COMMAND;
+	else if (keys[0] == '!')
+		*reg = VIS_REG_SHELL;
 	return keys+1;
 }
 

sam.c

@@ -492,9 +492,15 @@ static char *parse_text(const char **s) {
 	return buf.data;
 }
 
-static char *parse_shellcmd(const char **s) {
+static char *parse_shellcmd(Vis *vis, const char **s) {
 	skip_spaces(s);
-	return parse_until(s, "\n", NULL, false);
+	char *cmd = parse_until(s, "\n", NULL, false);
+	if (!cmd) {
+		const char *last_cmd = register_get(vis, &vis->registers[VIS_REG_SHELL], NULL);
+		return last_cmd ? strdup(last_cmd) : NULL;
+	}
+	register_put0(vis, &vis->registers[VIS_REG_SHELL], cmd);
+	return cmd;
 }
 
 static void parse_argv(const char **s, const char *argv[], size_t maxarg) {
@@ -746,7 +752,7 @@ static Command *command_parse(Vis *vis, const char **s, int level, enum SamError
 		}
 	}
 
-	if (cmddef->flags & CMD_SHELL && !(cmd->argv[1] = parse_shellcmd(s))) {
+	if (cmddef->flags & CMD_SHELL && !(cmd->argv[1] = parse_shellcmd(vis, s))) {
 		*err = SAM_ERR_SHELL;
 		goto fail;
 	}

vis.h

@@ -398,6 +398,7 @@ enum VisRegister {
 	VIS_MACRO_REPEAT,        /* copy of the above macro once the recording is finished */
 	VIS_REG_SEARCH,  /* last used search pattern "/ */
 	VIS_REG_COMMAND, /* last used :-command ": */
+	VIS_REG_SHELL, /* last used shell command given to either <, >, |, or ! */
 	VIS_REG_INVALID, /* has to be the last 'real' register */
 	VIS_REG_A, VIS_REG_B, VIS_REG_C, VIS_REG_D, VIS_REG_E,
 	VIS_REG_F, VIS_REG_G, VIS_REG_H, VIS_REG_I, VIS_REG_J,