Gitbase support for low-level git functions

[ajnavarro]

Related with #676

[kuba--]

Shall we mark it as blocked till we resolve #676 ?

[erizocosmico]

Can we add to the issue a bit more of information as to what is a "low-level git UDF"?

[ajnavarro]

I don't know exactly what they are, but the idea here is to have UDFs that has access directly to the repository instance, to do more complicated things.

[erizocosmico]

So just like UDFs but get another extra param that it's the repo?

[mcuadros]

Yes, we can work on this already, we need to access the repository (from sql.Context in the UDFs), for sure this will be need it.

[ajnavarro]

I would say to close this issue. Right now we are able to access the repository pool inside the context from any Expression. The only thing you need to do is send the repository_id as a function parameter, to be able to retrieve the needed repo.

[smola]

I changed the title from UDF to functions, which is what we originally meant, even if we said UDFs. The case for this is being able to write functions like DIFF, where you need to access the repo and you don't have all info as expression inputs (only repository_id and, for example, commit_hash). As far as I know, this can already be done with the context, but it would be good to document it. Once we have at least one function like DIFF working, I think we can consider this as fully solved.

[ajnavarro]

#811