pack-objects.c: fix some global variable abuse and memory leaks

pack-objects.c: fix some global variable abuse and memory leaks

To keep things well layered, sha1close() now returns the file descriptor
when it doesn't close the file.

An ugly cast was added to the return of write_idx_file() to avoid a
warning.  A proper fix will come separately.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>