submodule--helper: introduce add-clone subcommand

submodule--helper: introduce add-clone subcommand

Let's add a new "add-clone" subcommand to `git submodule--helper` with
the goal of converting part of the shell code in git-submodule.sh
related to `git submodule add` into C code. This new subcommand clones
the repository that is to be added, and checks out to the appropriate
branch.

This is meant to be a faithful conversion that leaves the behaviour of
'cmd_add()' script unchanged.

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