R1:0b3d10c3da91

Enforce sensible, unique clone/checkout names for repositories

Summary:
Fixes T7938.

- Primarily, users can currently shoot themselves in the foot by putting `../../etc/passwd` and other similar nonsense in these fields (this is not dangerous, but also does not work). Require sensible names.
- Enforce uniqueness so these names can be used in URIs and as identifiers in the future.
- (This doesn't start actually using them for anything fancy yet.)

Test Plan:
- Gave several repositories clone names…