packfile: close multi-pack-index in close_all_packs

Whenever we delete pack-files from the object directory, we need to also delete the multi-pack-index that may refer to those objects. Sometimes, this connection is obvious, like during a repack. Other times, this is less obvious, like when gc calls a repack command and then does other actions on the objects, like write a commit-graph file. The pattern we use to avoid out-of-date in-memory packed_git structs is to call close_all_packs(). This should also call close_midx(). Since we already pass an object store to close_all_packs(), this is a nicely scoped operation. This fixes a test failure when running t6500-gc.sh with GIT_TEST_MULTI_PACK_INDEX=1. Reported-by: Szeder Gábor <szeder.dev@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>