builtin/verify-commit: stop using `the_repository`

builtin/verify-commit: stop using `the_repository`

Remove the_repository global variable in favor of the repository
argument that gets passed in "builtin/verify-commit.c".

When `-h` is passed to the command outside a Git repository, the
`run_builtin()` will call the `cmd_verify_commit()` function with `repo`
set to NULL and then early in the function, `parse_options()` call will
give the options help and exit.

Pass the repository available in the calling context to `verify_commit()`
to remove it's dependency on the global `the_repository` variable.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>