Create helper functions to read the global magic environment variables
in additon to factoring out the global magic gathering logic into its
own function.
Signed-off-by: Brandon Williams <bmwill@google.com> Reviewed-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>