Enhancement: only open connection once for `init` command using sftp backend

The `init` command using the sftp backend used to connect twice to the
repository. This can be inconvenient if the user must enter a password or cause
`init` to fail if the server does not correctly close the first sftp
connection.

This has been fixed by reusing the initial sftp connection.

https://github.com/restic/restic/issues/2152
https://github.com/restic/restic/pull/3882
