builtin/verify-tag: stop using `the_repository`

builtin/verify-tag: stop using `the_repository`

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

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

Mentored-by: Christian Couder <redacted>
Signed-off-by: Usman Akinyemi <redacted>
Signed-off-by: Junio C Hamano <redacted>