R1:bf1cbc2499c7

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…