container: Fix importing to archive repos

[cgwalters]

Right now this fails due to https://github.com/ostreedev/ostree/blob/a113b2be3f597c2e4368f840063da70389e26fb0/src/libostree/ostree-repo-commit.c#L2745

Perhaps the simplest hack is to spawn a helper thread which writes the tar stream to a pipe, and then point a GUnixInputStream at the read side of the pipe

[dbnicholson]

I'd love if the root problem of pulling into an archive repo was fixed since I've run into that limitation in the past. Not a huge deal, though.

[cgwalters]

@dbnicholson This only applies to containers, are you using that now?

[dbnicholson]

@dbnicholson This only applies to containers, are you using that now?

Ah, no I'm not. There's some other feature that's not supported with archive mode repos, but I can't recall what it is. Carry on!