[API] Missing commit details functionality

Created by: vetruvet

There needs to be some functionality in the API for accessing commit details. Currently, you can list commits in a branch and get the HEAD commit for a branch or tag, but there is no way to get the details of a single commit.

Something like GET /projects/:id/repository/commits/:sha to get commit details: author and committer (name and email), authored and committed date, message, list of modified files, and diff stats (lines +/- per file). There is no way to get this information using the current API functions as far as I can see.

On an semi-related note, it is unclear what the created_at field represents in the commit list - is it the authored date, the committed date, or the pushed date (when the server received the commit)?