git: add a test for not keeping midx files open

This test creates a few dummy idx files, generates an midx,
queries a PackIdxList for a non-existent object, unlinks the
midx and checks that we still have it open, but that we close
it at PackIdxList::refresh now.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>