When a reference buffer is used multiple times then its index can be
computed only once and reused multiple times.  This patch adds an extra
pointer to a pointer argument (from_index) to diff_delta() for this.

If from_index is NULL then everything is like before.

If from_index is non NULL and *from_index is NULL then the index is
created and its location stored to *from_index.  In this case the caller
has the responsibility to free the memory pointed to by *from_index.

If from_index and *from_index are non NULL then the index is reused as
is.

This currently saves about 10% of CPU time to repack the git archive.

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