http-fetch: Abort requests for objects which arrived in packs

http-fetch: Abort requests for objects which arrived in packs

In fetch_object, there's a call to release an object request if the
object mysteriously arrived, say in a pack.  Unfortunately, the fetch
attempt for this object might already be in progress, and we'll leak the
descriptor.  Instead, try to tidy away the request.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>