I think the fact that git_value_fn returns 'const char*' is a
concern as it means we cannot return an allocated string.

Yeah, I was thinking about that. It is not a problem by now, but
after the acceptance of this patchset I'll start to work in the
`path.*` fields and that would be a problem.

It would be better to pass a 'struct strbuf' to the callback and
write the value to that instead.

When I was working on this version I was thinking about it, but I
left the way it is to make it simpler, and refactor it when
implementing the `path.*` fields. But now I think it would be
better to change it in v6.