completion: implement and use --list-cmds=main,others

This is part of the effort to break down and provide commands by
category in machine-readable form. This could be helpful later on when
completion script switches to use --list-cmds for selecting
completable commands. It would be much easier for the user to choose
to complete _all_ commands instead of the default selection by passing
different values to --list-cmds in git-completino.bash.

While at there, replace "git help -a" in git-completion.bash with
--list-cmds since it's better suited for this task.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.h

@@ -1,6 +1,8 @@
 #ifndef HELP_H
 #define HELP_H
 
+struct string_list;
+
 struct cmdnames {
 	int alloc;
 	int cnt;
@@ -17,6 +19,8 @@ static inline void mput_char(char c, unsigned int num)
 }
 
 extern void list_common_cmds_help(void);
+extern void list_all_main_cmds(struct string_list *list);
+extern void list_all_other_cmds(struct string_list *list);
 extern const char *help_unknown_cmd(const char *cmd);
 extern void load_command_list(const char *prefix,
 			      struct cmdnames *main_cmds,