run-command API users: use "env" not "env_array" in comments & names

run-command API users: use "env" not "env_array" in comments & names

Follow-up on a preceding commit which changed all references to the
"env_array" when referring to the "struct child_process" member. These
changes are all unnecessary for the compiler, but help the code's
human readers.

All the comments that referred to "env_array" have now been updated,
as well as function names and variables that had "env_array" in their
name, they now refer to "env".

In addition the "out" name for the submodule.h prototype was
inconsistent with the function definition's use of "env_array" in
submodule.c. Both of them use "env" now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>