dim: url_to_remote can't normally fail

Since commit cad37e19 ("dim: auto-add remotes") url_to_remote can't
really fail. Same for repo_to_remote when the repo exists. Redirecting
their output when the remote isn't there leads to url_to_remote waiting
for user input without prompting, giving an appearance of a hang.

While at it, change the exit to a return. set -e at the top takes care
of aborting.

Using git worktrees is now practically required because we need all the
remotes everywhere. Worktrees have been the default for a while now, and
you have to go out of your way to set dim up without worktrees. You can
still get by without, but it'll be clumsy.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>