fetch: after refetch, encourage auto gc repacking

fetch: after refetch, encourage auto gc repacking

After invoking `fetch --refetch`, the object db will likely contain many
duplicate objects. If auto-maintenance is enabled, invoke it with
appropriate settings to encourage repacking/consolidation.

* gc.autoPackLimit: unless this is set to 0 (disabled), override the
  value to 1 to force pack consolidation.
* maintenance.incremental-repack.auto: unless this is set to 0, override
  the value to -1 to force incremental repacking.

Signed-off-by: Robert Coup <robert@coup.net.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>