Multi-file editor should load file list before last commit

The multi-file editor currently loads files and last_commit data in the same request. Retrieving last_commit data is slow. This makes loading the file list slow all the time.

Proposal

• file list always be loaded first
• last_commit data should be loaded second

Edge cases:

• If a file is opened directly (e.g. https://gitlab.com/gitlab-org/gitlab-ce/blob/master/README.md), the tree rendered to the left of the blob only requires the file list be loaded. last_commit data should only be loaded when the blob view is dismissed.