maintenance: simplify prefetch logic

maintenance: simplify prefetch logic

The previous logic filled a string list with the names of each remote,
but instead we could simply run the appropriate 'git fetch' data
directly in the remote iterator. Do this for reduced code size, but also
because it sets up an upcoming change to use the remote's refspec. This
data is accessible from the 'struct remote' data that is now accessible
in fetch_remote().

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