submodule--helper: run update procedures from C

submodule--helper: run update procedures from C

Add a new submodule--helper subcommand `run-update-procedure` that runs
the update procedure if the SHA1 of the submodule does not match what
the superproject expects.

This is an intermediate change that works towards total conversion of
`submodule update` from shell to C.

Specific error codes are returned so that the shell script calling the
subcommand can take a decision on the control flow, and preserve the
error messages across subsequent recursive calls of `cmd_update`.

This change is more focused on doing a faithful conversion, so for now we
are not too concerned with trying to reduce subprocess spawns.

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