Digest changeset anchors into purely alphanumeric strings

Summary:
Ref T13045. See that task for discussion.

This replaces digestForIndex() with a "clever" algorithm in digestForAnchor(). The new digest is the same as digestForIndex() except when the original output was "." or "_". In those cases, a replacement character is selected based on entropy accumulated by the digest function as it iterates through the string.

Test Plan: Added unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13045

Differential Revision: https://secure.phabricator.com/D18909