In C code we have the luxury of having constants for all the important
things that are hard coded. This is the only place in C that hard codes
the git directory environment variable, so fix it.
Signed-off-by: Stefan Beller <sbeller@google.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>