Standardize commit diff api url, change blob api url, add get single commit

Use "/commits/:sha/diff" as opposed to "/commit/:sha", keeping in line
with existing api urls (e.g. "/projects/:id", etc.)

Fix 500 error resulting from a diff api call with an invalid commit hash

Move "/commits/:sha/blob" to "/blobs/:sha", leaving the old path for
backwards compatibility.

Add ability to get a single commit via "/commits/:sha"
Merge pull request #4906 from alexvh/standardize_diff_api_url

Standardize recently added commit diff api url