Avoid mmapping small files while using packed refs (especially ones
with zero size, which would cause later munmap() to fail).
* kg/packed-ref-cache-fix:
packed_ref_cache: don't use mmap() for small files
load_contents(): don't try to mmap an empty file
packed_ref_iterator_begin(): make optimization more general
find_reference_location(): make function safe for empty snapshots
create_snapshot(): use `xmemdupz()` rather than a strbuf
struct snapshot: store `start` rather than `header_len`