environment: add GIT_PREFIX to local_repo_env

environment: add GIT_PREFIX to local_repo_env

The GIT_PREFIX variable is set based on our location within
the working tree. It should therefore be cleared whenever
GIT_WORK_TREE is cleared.

In practice, this doesn't cause any bugs, because none of
the sub-programs we invoke with local_repo_env cleared
actually care about GIT_PREFIX. But this is the right thing
to do, and future proofs us against that assumption changing.

While we're at it, let's define a GIT_PREFIX_ENVIRONMENT
macro; this avoids repetition of the string literal, which
can help catch any spelling mistakes in the code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>