Previously, it would not be possible to call start_command twice for the
same struct child_process that has env set.
The fix is achieved by moving the loop that modifies the environment block
into a helper function. This also allows us to make two other helper
functions static.
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>