1. Don't use case statements for dispatch anymore. This leads to a lot
of duplication, and makes the logic harder to follow.
2. Remove duplicated logic.
- For example, the `can_push_to_branch?` exists, but we also have a
different way of checking the same condition within `change_access_check`.
- This kind of duplication is removed, and the `can_push_to_branch?`
method is used in both places.
3. Move checks returning true/false to `UserAccess`.
- All public methods in `GitAccess` now return an instance of
`GitAccessStatus`. Previously, some methods would return
true/false as well, which was confusing.
- It makes sense for these kinds of checks to be at the level of a
user, so the `UserAccess` class was repurposed for this. The prior
`UserAccess.allowed?` classmethod is converted into an instance
method.
- All external uses of these checks have been migrated to use the
`UserAccess` class
4. Move the "change_access_check" into a separate class.
- Create the `GitAccess::ChangeAccessCheck` class to run these
checks, which are quite substantial.
- `ChangeAccessCheck` returns an instance of `GitAccessStatus` as
well.
5. Break out the boolean logic in `ChangeAccessCheck` into `if/else`
chains - this seems more readable.
6. I can understand that this might look like overkill for !4892, but I
think this is a good opportunity to clean it up.
- http://martinfowler.com/bliki/OpportunisticRefactoring.html
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