command-list.txt: add the common groups block

command-list.txt: add the common groups block

The ultimate goal is for "git help" to display common commands in
groups rather than alphabetically. As a first step, define the
groups in a new block, and then assign a group to each
common command.

Add a block at the beginning of command-list.txt:

    init         start a working area (see also: git help tutorial)
    worktree     work on the current change (see also:[...]
    info         examine the history and state (see also: git [...]
    history      grow, mark and tweak your history
    remote       collaborate (see also: git help workflows)

storing information about common commands group, then map each common
command to a group:

    git-add          mainporcelain        common worktree

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by:  Emma Jane Hogbin Westby <emma.westby@gmail.com>
Signed-off-by: Sébastien Guimmara <sebastien.guimmara@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>