Setting version metadata doesn't preserve the pre-release version

An example:
```sh
❯ cargo set-version -p package -m 11.ge8fe5ff --dry-run
Upgrading package from 0.1.17-beta.3 to 0.1.17+11.ge8fe5ff
warning: aborting set-version due to dry run
❯ cargo set-version --version
cargo-edit-set-version 0.12.3
```

I'd imagine the version should change to `0.1.17-beta.3+11.ge8fe5ff`. Is this intentional or a bug?

Reproduce the dry-run with `cargo set-version -p package -m 11.ge8fe5ff --dry-run`, then trace the `set-version` handling of the `-m` metadata and prerelease components. Done means the reported change preserves `0.1.17-beta.3` and produces `0.1.17-beta.3+11.ge8fe5ff`.

Written by the indexing model from the issue text.