builtin/repack.c: remove ref snapshotting from builtin

builtin/repack.c: remove ref snapshotting from builtin

When writing a MIDX, 'git repack' takes a snapshot of the repository's
references and writes the result out to a file, which it then passes to
'git multi-pack-index write' via the '--refs-snapshot'.

This is done in order to make bitmap selections with respect to what we
are packing, thus avoiding a race where an incoming reference update
causes us to try and write a bitmap for a commit not present in the
MIDX.

Extract this functionality out into a new repack-midx.c compilation
unit, and expose the necessary functions via the repack.h API.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>