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., mentions).
• Grepped for removed getShortName().

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14990