[RFC PATCH 0/5] ref-filter: add new formatting options

Add formatting options %(objectsize:disk) and %(deltabase), as in cat-file command.

I can not test %(deltabase) properly (I mean, I want to have test with meaningful deltabase in the result - now we have only with zeros). I tested it manually on my git repo, and I have not-null deltabases there. We have "t/t1006-cat-file.sh" with similar case, but it is about blobs. ref-filter does not work with blobs, I need to write test about refs, and I feel that I can't catch the idea (and it is hard for me to write in Shell).

Finally, I want to remove formatting logic in cat-file and use functions from ref-filter (we are almost there, so many work was done for this). I had an idea to make this migration in this patch (and stop worrying about bad tests about deltabase: we already have such test for cat-file and hopefully that could be enough). But I have another question there. cat-file has one more formatting option: "rest" [1]. Do we want such formatting option in ref-filter? It's easier for me to support that in ref-filter than to leave it only specifically for cat-file.

Thank you!

[1] https://git-scm.com/docs/git-cat-file#git-cat-file-coderestcode