Truncate SHA-1 indexes

Truncate SHA-1 indexes

Per Asher Feldman's suggestion on wikitech-l, truncate DB indexes which
hold SHA-1 values, so that the space usage is reduced at the expense of
a very small overhead when selecting values that have colliding
truncated hashes.

Change-Id: I059de4f7bba9a949cf0a93a57df32226c41d8ce0