On every push received by the post-receive hook, the WikiPushService is
run on the changes. This modifies that file to look at the changes in
the master branch (well, wiki repo default branch) and create any wiki
events necessary.

These actions are guarded by two feature flags:

 - :wiki_events
    This prevents wiki pushes creating events while the wiki_events flag is
    off. This flag is the same flag used by the WikiPage services.
 - :wiki_events_on_git_push, (scoped to project)
    Specific feature flag for git-access

- WikiPushService::Change

Most of the state is in the change class, combining the change with the
raw diff change, and this extraction tidies
things up a lot.

Logic repeated between EE and core is refactored to be consistent.

- Add WikiPages::EventCreateService

This service wraps EventCreateService and manages the instantiation of
WikiPage::Meta objects. A spec is added to verify this.

This verifies the three public methods for this class
- last_known_slug
- page
- event_action

The number of changes we will process is subject to a safety limit.