R1:b848ab87b3d5

Introduce "local names" for commits

Summary: Ref T4245. Full commit display names (like `rPaaaa`) are going to be obnoxious soon in some cases (e.g., `rPaaaa` becomes `R123:aaaa`, which is much uglier) so reduce how often we show the repository in cases where it isn't really necessary to include it.

Test Plan:
- Saw no more `rX` on repository list view for Git/Mercurial (still present for Subversion).
- Saw no more `rX` on various repository detail views, except when referencing other commits (e.g.