fetch: make the --prune-tags work with <url>

fetch: make the --prune-tags work with <url>

Make the new --prune-tags option work properly when git-fetch is
invoked with a <url> parameter instead of a <remote name>
parameter.

This change is split off from the introduction of --prune-tags due to
the relative complexity of munging the incoming argv, which is easier
to review as a separate change.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>