dim_setup: Use "repo" for remote default name

Instead of slicing the url and deriving the default name for a remote
from there, use a new url_to_repo function to find the name from the
nightly.conf. This fixes the default name for the drm-xe repo being
called "kernel" since the url is gitlab.freedesktop.org/drm/xe/kernel.git

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>