generate-cmdlist.sh: collect config from all config.txt files

generate-cmdlist.sh: collect config from all config.txt files

This script uses Documentation/config.txt as input for "git help
--config" and "git config" completion but it misses the fact that
config.txt includes other txt files. Include all *config.txt as input
when scanning for config keys. This could produce false positives, but
as long as we stick to the blah-config.txt naming convention, we
should be ok.

While at there, move diff.* from config.txt to diff-config.txt where
all other diff config keys are.

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