Merge branch 'ds/midx-too-many-packs'

The code to generate the multi-pack idx file was not prepared to
see too many packfiles and ran out of open file descriptor, which
has been corrected.

* ds/midx-too-many-packs:
midx: add packs to packed_git linked list
midx: pass a repository pointer

diff --cc builtin/pack-objects.c
Simple merge

diff --cc object-store.h
Simple merge

diff --cc packfile.c
Simple merge

diff --cc sha1-name.c
Simple merge