Fix a dirname() edge case in Diffusion

Summary:
dirname('x') returns '.', not '/'; this caused some issues for repositories with
files at the root.

There are some cases in the parsers where I should probably swap this out too
but I'll wait until I'm doing some more rigorous testing since that stuff is a
bit fragile and this fixes an immediate issue.

Test Plan: Ran unit tests. Viewed a file at root level in a test repository.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, nh

Differential Revision: 932