help: do not expect built-in commands to be hardlinked

When building with SKIP_DASHED_BUILT_INS=YesPlease, the built-in
commands are no longer present in the `PATH` as hardlinks to `git`.

As a consequence, `load_command_list()` needs to be taught to find the
names of the built-in commands from elsewhere.

This only affected the output of `git --list-cmds=main`, but not the
output of `git help -a` because the latter includes the built-in
commands by virtue of them being listed in command-list.txt.

The bug was detected via a patch series that turns the merge strategies
included in Git into built-in commands: `git merge -s help` relies on
`load_command_list()` to determine the list of available merge
strategies.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -637,6 +637,25 @@ static void list_builtins(struct string_list *out, unsigned int exclude_option)
 	}
 }
 
+void load_builtin_commands(const char *prefix, struct cmdnames *cmds)
+{
+	const char *name;
+	int i;
+
+	/*
+	 * Callers can ask for a subset of the commands based on a certain
+	 * prefix, which is then dropped from the added names. The names in
+	 * the `commands[]` array do not have the `git-` prefix, though,
+	 * therefore we must expect the `prefix` to at least start with `git-`.
+	 */
+	if (!skip_prefix(prefix, "git-", &prefix))
+		BUG("prefix '%s' must start with 'git-'", prefix);
+
+	for (i = 0; i < ARRAY_SIZE(commands); i++)
+		if (skip_prefix(commands[i].cmd, prefix, &name))
+			add_cmdname(cmds, name, strlen(name));
+}
+
 #ifdef STRIP_EXTENSION
 static void strip_extension(const char **argv)
 {

help.c

@@ -263,6 +263,8 @@ void load_command_list(const char *prefix,
 	const char *env_path = getenv("PATH");
 	const char *exec_path = git_exec_path();
 
+	load_builtin_commands(prefix, main_cmds);
+
 	if (exec_path) {
 		list_commands_in_dir(main_cmds, exec_path, prefix);
 		QSORT(main_cmds->names, main_cmds->cnt, cmdname_compare);

help.h

@@ -32,6 +32,7 @@ const char *help_unknown_cmd(const char *cmd);
 void load_command_list(const char *prefix,
 		       struct cmdnames *main_cmds,
 		       struct cmdnames *other_cmds);
+void load_builtin_commands(const char *prefix, struct cmdnames *cmds);
 void add_cmdname(struct cmdnames *cmds, const char *name, int len);
 /* Here we require that excludes is a sorted list. */
 void exclude_cmds(struct cmdnames *cmds, struct cmdnames *excludes);