run-command: refactor subprocess env preparation

run-command: refactor subprocess env preparation

submodule.c has functionality that prepares the environment for running
a subprocess in a new repo. The lazy-fetching code (used in partial
clones) will need this in a subsequent commit, so move it to a more
central location.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>