Don't let users pick "whatever.git" as a repository short name, make "." work

Summary:
Fixes T11902.

• Periods now work in short names.
• If you try to name something ".git", no dice.

Test Plan:

• Tried to name something "quack.git", was politely rejected.
• Named something "quack.notgit", and it worked fine.
• Cloned Mercurial and Git repositories over SSH with ".git" and non-".git" variants without hitting any issues.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11902

Differential Revision: https://secure.phabricator.com/D16908