config: correct bad boolean env value error message

config: correct bad boolean env value error message

An incorrectly defined boolean environment value would result in the
following error message:

bad boolean config value '%s' for '%s'

This is a misnomer since environment value != config value. Instead of
calling git_config_bool() to parse the environment value, mimic the
functionality inside of git_config_bool() but with the correct error
message.

Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>