help -a: improve and make --verbose default

help -a: improve and make --verbose default

When you type "git help" (or just "git") you are greeted with a list
with commonly used commands and their short description and are
suggested to use "git help -a" or "git help -g" for more details.

"git help -av" would be more friendly and inline with what is shown
with "git help" since it shows list of commands with description as
well, and commands are properly grouped.

"help -av" does not show everything "help -a" shows though. Add
external command section in "help -av" for this. While at there, add a
section for aliases as well (until now aliases have no UI, just "git
config").

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