Add 'const' where appropriate to index handling functions

Add 'const' where appropriate to index handling functions

This is in an effort to make the source index of 'unpack_trees()' as
being const, and thus making the compiler help us verify that we only
access it for reading.

The constification also extended to some of the hashing helpers that get
called indirectly.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>