Expand abbreviated Mercurial hashes to full hashes

Summary:
If you go to `/rXnnnn` in Git, we expand the hash. If you go to `/rXnnnn` in Mercurial, we give you a confusing error message.

Reconcile Mercurial behavior with Git. Fixes T2265.

Test Plan: Viewed partial hash, full hash commit in Diffusion. Viewed very short hash, got reasonable behaviors.

Reviewers: btrahan, tido

Reviewed By: tido

CC: aran

Maniphest Tasks: T2265

Differential Revision: https://secure.phabricator.com/D4330