Add last_commit_sha to the commit API

If a file is edited on the server while user is making changes using the multi-file editor:

the commit should be rejected by the server
an informative error message shown to the user
client side changes should not be lost (allowing the user to commit them to a new branch)

The commit API should reject the commit if last_commit_sha is different to the last_commit_sha on the server

Add last_commit_sha to commit API
Reject commit if last_commit_sha doesn't match the last_commit_sha on the server