Release checks public registry even with publish set to a private one

[bobrik]

See #732 and #735.

If a crate has the following set in [package] in Cargo.toml:

publish = ["myregistry"]

Here's the documentation:

• https://doc.rust-lang.org/cargo/reference/manifest.html#the-publish-field

If publish array contains a single registry, cargo publish command will use it when --registry flag is not specified.

It would be good if cargo-release followed the same heuristic. Currently one needs to pass --registry manually, otherwise the public registry is checked, which might not be accessible and it leaks crate names (#732).

[epage]

The key quote to keep in mind from that documentation:

If publish array contains a single registry, cargo publish command will use it when --registry flag is not specified.