sha1: use the real SHA-1 algorithm
The sha1collisiondetection library by default performs its own "safe-hash"
algorithm, which is not actually the same as the normal SHA-1 algorithm.
For most inputs the output will be the actual SHA-1 hash, but if the input
happens to contain one of several known SHA-1 disturbance vectors this code
returns an alternate "safe-hash" output instead.
This updates the code to explicitly disable "safe-hash" mode and collision
detection, so that we always get actual SHA-1 hashes.
Test Plan: Ran the unit tests.
Reviewers: #fbhgext, ryanmce
Reviewed By: #fbhgext, ryanmce
Differential Revision: https://phab.mercurial-scm.org/D284