Support for git_filter_*

[samlh]

Ideally, this would include allowing creation of custom filters (trait name recommendations appreciated).

Specifically, I think I'll need:

• git_filter
• git_filter_register
• git_blob_filtered_content

I may work on this for my current project.

[samlh]

As it turns out, it was easier to implement git-lfs support for my project without using libgit2's filter support, so I no longer have plans to work on this.

Please feel free to close this or keep it open as you prefer.

[extrawurst]

@samlh stumbled over this on my search to implement git-lfs myself. how did you end up supporting it?

[samlh]

@extrawurst It's been a while, but I've pushed up the code here:
https://github.com/samlh/gitblob/blob/97a897da973af6b0a5548469fcd5e081936074cd/src/main.rs#L255-L276

I seem to recall the code is making some assumptions about the repos it has to work with, but I'll admit I've forgotten what the assumptions were 😄.

I hope it helps regardless!