Allow diffusion.getcommits to lookup commits by short reference

Summary:
Enable "rPf1d43bc" to lookup "rPf1d43bc3c56491b4de208fc4176fc4772436afe5".

Test Plan:
Looked up full commits by short reference.

Reviewed By: mroch
Reviewers: mroch, tuomaspelkonen, jungejason, aran
Commenters: jungejason
CC: aran, mroch, jungejason
Differential Revision: 304