merge-ours: drop USE_THE_REPOSITORY_VARIABLE

merge-ours: drop USE_THE_REPOSITORY_VARIABLE

The merge-ours built-in uses the `the_repository` global to access
the repository. The project is moving away from this global in favor
of the `repo` parameter that is passed to each built-in command.
Since merge-ours is registered with RUN_SETUP, `repo` is guaranteed
to be non-NULL and can be used directly.

Drop the USE_THE_REPOSITORY_VARIABLE macro and use `repo` throughout.

While at it, remove a stray double blank line between the #include
block and the usage string.

Signed-off-by: Sam Bostock <sam@sambostock.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>