many cleanups to sha1_file.c

many cleanups to sha1_file.c

Those cleanups are mainly to set the table for the support of deltas
with base objects referenced by offsets instead of sha1.  This means
that many pack lookup functions are converted to take a pack/offset
tuple instead of a sha1.

This eliminates many struct pack_entry usages since this structure
carried redundent information in many cases, and it increased stack
footprint needlessly for a couple recursively called functions that used
to declare a local copy of it for every recursion loop.

In the process, packed_object_info_detail() has been reorganized as well
so to look much saner and more amenable to deltas with offset support.

Finally the appropriate adjustments have been made to functions that
depend on the above changes.  But there is no functionality changes yet
simply some code refactoring at this point.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>