submodule: port subcommand 'set-branch' from shell to C
Convert submodule subcommand 'set-branch' to a builtin and call it via
'git-submodule.sh'.
Mentored-by: Christian Couder <chriscool@tuxfamily.org> Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com> Helped-by: Denton Liu <liu.denton@gmail.com> Helped-by: Eric Sunshine <sunshine@sunshineco.com> Helped-by: Đoàn Trần Công Danh <congdanhqx@gmail.com> Signed-off-by: Shourya Shukla <shouryashukla.oo@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>