upload-archive: use regular "struct child_process" pattern

upload-archive: use regular "struct child_process" pattern

This pattern added [1] in seems to have been intentional, but since
[2] and [3] we've wanted do initialization of what's now the "struct
strvec" "args" and "env_array" members. Let's not trample on that
initialization here.

1. 1bc01efed17 (upload-archive: use start_command instead of fork,
   2011-11-19)
2. c460c0ecdca (run-command: store an optional argv_array, 2014-05-15)
3. 9a583dc39e (run-command: add env_array, an optional argv_array for
   env, 2014-10-19)

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