http.c: Don't store destination name in request structures

http.c: Don't store destination name in request structures

The destination name within the object store is easily computed
on demand, reusing a static buffer held by sha1_file.c.  We don't
need to copy the entire path into the request structure for safe
keeping, when it can be easily reformatted after the download has
been completed.

This reduces the size of the per-request structure, and removes
yet another PATH_MAX based limit.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>