Merge branch 'lf/read-blob-data-from-index'

Reduce duplicated code between convert.c and attr.c.

* lf/read-blob-data-from-index:
  convert.c: remove duplicate code
  read_blob_data_from_index(): optionally return the size of blob data
  attr.c: extract read_index_data() as read_blob_data_from_index()

diff --cc attr.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc convert.c
Simple merge

diff --cc read-cache.c
Simple merge