Replace parse_blob() with an explanatory comment

Replace parse_blob() with an explanatory comment

parse_blob() has never actually been used; it has served simply to
avoid having a confusing gap in the API. Instead of leaving it, put in
a comment that explains what "parsing a blob" entails (making sure the
object is actually readable), and why code might care whether a blob
has been parsed or not.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>