help: add "-a --verbose" to list all commands with synopsis

help: add "-a --verbose" to list all commands with synopsis

This lists all recognized commands [1] by category. The group order
follows closely git.txt.

[1] We may actually show commands that are not built (e.g. if you set
NO_PERL you don't have git-instaweb but it's still listed here). I
ignore the problem because on Linux a git package could be split
anyway. The "git-core" package may not contain git-instaweb even if
it's built because it may end up in a separate package. We can't know
anyway.

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