t3701: stop using `env` in force_color()

t3701: stop using `env` in force_color()

In a future patch, we plan on making the test_must_fail()-family of
functions accept only git commands. Even though force_color() wraps an
invocation of `env git`, test_must_fail() will not be able to figure
this out since it will assume that force_color() is just some random
function which is disallowed.

Instead of using `env` in force_color() (which does not support shell
functions), export the environment variables in a subshell. Write the
invocation as `force_color test_must_fail git ...` since shell functions
are now supported.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>