Document why header parsing won't exceed a window.

Document why header parsing won't exceed a window.

When we parse the object header or the delta base reference we
don't bother to loop over use_pack() calls.  The reason we don't
need to bother with calling use_pack for each byte accessed is that
use_pack will always promise us at least 20 bytes (really the hash
size) after the offset.  This promise from use_pack simplifies a
lot of code in the header parsing logic, as well as helps out the
zlib library by ensuring there's always some data for it to consume
during an inflate call.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>