submodule--helper: run update using child process struct

submodule--helper: run update using child process struct

We switch to using the run-command API function that takes a
'struct child process', since we are using a lot of the options. This
will also make it simple to switch over to using 'capture_command()'
when we start handling the output of the command completely in C.

Mentored-by: Christian Couder <redacted>
Mentored-by: Shourya Shukla <redacted>
Signed-off-by: Atharva Raykar <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>