attr.c: extract read_index_data() as read_blob_data_from_index()

Extract the read_index_data() function from attr.c and move it to
read-cache.c; rename it to read_blob_data_from_index() and update
the function signature of it to align better with index/cache API
functions.

This allows for reusing the function in convert.c later.

Signed-off-by: Lukas Fleischer <git@cryptocrack.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
read_blob_data_from_index(): optionally return the size of blob data

This allows for optionally getting the size of the returned data and
will be used in a follow-up patch.

Signed-off-by: Lukas Fleischer <git@cryptocrack.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
convert.c: remove duplicate code

The has_cr_in_index() function is an almost 1:1 copy of
read_blob_data_from_index() with some additions.  Use the
latter instead of using copy-pasted code.

Signed-off-by: Lukas Fleischer <git@cryptocrack.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'lf/read-blob-data-from-index' into next

* 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()