refs: read loose references lazily

refs: read loose references lazily

Instead of reading the whole directory of loose references the first
time any are needed, only read them on demand, one directory at a
time.

Use a new ref_entry flag bit REF_INCOMPLETE to indicate that the entry
represents a REF_DIR that hasn't been read yet.  Whenever any entries
from such a directory are needed, read all of the loose references
from that directory.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>