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: NLukas Fleischer <git@cryptocrack.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>