No more check function

[ammeep]

This PR removes the need to supply the check callback when registering a Filter with the library.

For example if you want to install a filter with libgit2 with the name foo you would register that filter with libgit2sharp and it would be applied to any file that matched the path spec found in the gitattributes file. For example *.txt filter=foo

I naively thought that we needed to implement custom check logic for filters (the piece that actually matches the entry in the attributes file to the file being filtered) but it turns out this is already implemented in libgit2 https://github.com/libgit2/libgit2/blob/master/src/filter.c#L403 and if we simply do not provide the optional check callback, everything just works ™️

I've added an object FilterAttribute which allows you to define and register a filter in the correct format filter=name

[ammeep]

I would love some eyes on this @nulltoken.

Want to make sure it is sane before I merge it, squash down the register filter PR, and update libgit2 to the latest 💅

[Therzok]

Nit: Just use the linq inside the initializer list?