fetch: use ref_namespaces during prefetch

fetch: use ref_namespaces during prefetch

The "refs/prefetch/" namespace is used by 'git fetch --prefetch' as a
replacement of the destination of the refpsec for a remote. Git also
removes refspecs that include tags.

Instead of using string literals for the 'refs/tags/ and
'refs/prefetch/' namespaces, use the entries in the ref_namespaces
array.

This kind of change could be done in many places around the codebase,
but we are isolating only to this change because of the way the
refs/prefetch/ namespace somewhat motivated the creation of the
ref_namespaces array.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>