midx-write: pass down repository to `write_midx_file[_only]`

midx-write: pass down repository to `write_midx_file[_only]`

In a previous commit, we passed the repository field to all
subcommands in the `builtin/` directory. Utilize this to pass the
repository field down to the `write_midx_file[_only]` functions to
remove the usage of `the_repository` global variables.

With this, all usage of global variables in `midx-write.c` is removed,
hence, remove the `USE_THE_REPOSITORY_VARIABLE` guard from the file.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>