submodule--helper: convert the bulk of cmd_add() to C

submodule--helper: convert the bulk of cmd_add() to C

Introduce the 'add' subcommand to `submodule--helper.c` that does all
the work 'submodule add' past the parsing of flags.

We also remove the constness of the sm_path field of the `add_data`
struct. This is needed so that it can be modified by
normalize_path_copy().

As with the previous conversions, this is meant to be a faithful
conversion with no modification to the behaviour of `submodule add`.

Signed-off-by: Atharva Raykar <redacted>
Mentored-by: Christian Couder <redacted>
Helped-by: Kaartic Sivaraam <redacted>
Mentored-by: Shourya Shukla <redacted>
Based-on-patch-by: Shourya Shukla <redacted>
Based-on-patch-by: Prathamesh Chavan <redacted>
Signed-off-by: Junio C Hamano <redacted>