completion: support case-insensitive config vars

completion: support case-insensitive config vars

Config variables are case-insensitive but this case/esac construct is
case-sensitive by default. For bash v4, it'll be easy. For platforms
that are stuck with older versions, we need an external command, but
that is not that critical. And where this additional overhead matters
the most is Windows, but luckily Git for Windows ships with Bash v4.

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