Add support for nested prefabs and prefab variants

[FlaShG]

Investigate what needs to be done to properly support those new concepts, then implement that.

[Dak0r]

Hi, thank you for your work on this project! Is there a list of known issues with prefabs that make use of nesting / variants?

Edit: I did some quick testing: Some Info on nested/variants: Those work in the way, that the variants and prefabs with nested prefabs yml files only contain a reference to the original (or nested) prefab and a list of attributes that are overriden by it. So the concept is quite basic.

• Git Merge for Unity generally seems to work quite well with variants and and nested prefabs. Changes are always applied in the file where the conflict happened, meaning they are treated as overrides for if it happened in a variant or prefab that nests an other one (as expected!).

• Changes in the originals are correctly applied, if no override exists (as expected).

I didn't test it extensively, yet, but it seems to work good enough for basic conflicts 🙂

During my test I found a different issue related to RectTransforms: https://github.com/FlaShG/GitMerge-for-Unity/issues/67