Update partial HTTP transfers.

Update partial HTTP transfers.

Add the sanity checks discussed on the list with Nick Hengeveld in
<20050927000931.GA15615@reactrix.com>.

 * unlink of previous and rename from temp to previous can fail for
   reasons other than benign ones (missing previous and missing temp).
   Report these failures when we encounter them, to make diagnosing
   problems easier.

 * when rewinding the partially written result, make sure to
   truncate the file.

Also verify the pack after downloading by calling
verify_packfile().

Signed-off-by: Junio C Hamano <junkio@cox.net>