don't hang when copy uses a single connection

[MichaelEischer]

What does this PR change? What problem does it solve?

The copy command hangs when the source repository only uses a single backend connection. This is caused by reserving one connection for uploading, which is necessary to prevent deadlock for the prune command. However, copy uses different repositories, so there is reason to reserve a connection.

Was the change previously discussed in an issue or on the forum?

Fixes #3897

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.