builtin/prune: stop depending on 'the_repository'

builtin/prune: stop depending on 'the_repository'

Refactor builtin/prune.c to remove the dependency on the global
'the_repository'. Replace all the occurrences of 'the_repository' with
repo and thus remove the definition '#define
USE_THE_REPOSITORY_VARIABLE'. Also, add a test to make sure that 'git
prune -h' can be called when the repository is `NULL`.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>