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>

blob.h

@@ -13,6 +13,13 @@ struct blob *lookup_blob(const unsigned char *sha1);
 
 int parse_blob_buffer(struct blob *item, void *buffer, unsigned long size);
 
-int parse_blob(struct blob *item);
+/**
+ * Blobs do not contain references to other objects and do not have
+ * structured data that needs parsing. However, code may use the
+ * "parsed" bit in the struct object for a blob to determine whether
+ * its content has been found to actually be available, so
+ * parse_blob_buffer() is used (by object.c) to flag that the object
+ * has been read successfully from the database.
+ **/
 
 #endif /* BLOB_H */