Allow to specify a key for private registries

[l4l]

Tried to build a crate for private repo and it works fine. The fixes actually looks really awkward due to weird git2 interface, would love to hear any comments how to make it more readable.

Related PR in rustwide: rust-lang/rustwide#65
And a version update for crates-index-diff: Byron/crates-index-diff-rs#11

[jyn514]

Marking this as blocked on rust-lang/rustwide#65.

[gregvirgin]

Raising here as in rust-lang/rustwide#65, cloudsmith.io requires .git-credentials and won't support SSH keys.

[gregvirgin]

@l4l what private repo are you using?

[l4l]

I've been using self-hosted meuse quite some time ago. Current version of rustdoc supports only http auth (e.g http://login:password@my-server). Git credentials could be added separately and perhaps unrelated to this PR.

[l4l]

Although the blocked-by PR has been merged to rustwide I missed time slot for finishing this one before the git crate for crates-index-diff has been changed: Byron/crates-index-diff-rs@4cedf27. Currently it is using gix which if (I understand correctly)[https://github.com/Byron/gitoxide/issues/106) makes this patch obsolete because now keys in ~/.ssh could be used.