sha1_file: refactor read_object

read_object() and sha1_object_info_extended() both implement mechanisms
such as object replacement, retrying the packed store after failing to
find the object in the packed store then the loose store, and being able
to mark a packed object as bad and then retrying the whole process.
Consolidating these mechanisms would be a great help to maintainability.

Therefore, consolidate them by extending sha1_object_info_extended() to
support the functionality needed, and then modifying read_object() to
use sha1_object_info_extended().

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1827,6 +1827,7 @@ struct object_info {
 	off_t *disk_sizep;
 	unsigned char *delta_base_sha1;
 	struct strbuf *typename;
+	void **contentp;
 
 	/* Response */
 	enum {