Allow number in generated clone uri

Summary:
See https://discourse.phabricator-community.org/t/numerical-characters-are-stripped-from-diffusion-git-repository-name-in-the-uri/

Digits are often considered reasonable characters.

Test Plan: Looked at an ascii table.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, Sam2304, epriestley

Differential Revision: https://secure.phabricator.com/D19447