help: use command-list.txt for the source of guides
The help command currently hard codes the list of guides and their
summary in C. Let's move this list to command-list.txt. This lets us
extract summary lines from Documentation/git*.txt. This also
potentially lets us list guides in git.txt, but I'll leave that for
now.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>