workaround svn issue with svn+ssh scheme requiring to specify user@host and ignoring --username

See MRELEASE-35 for details.

Here's the patch: I fixed the tag and checkout commands. Other commands may need to be fixed for other purposes, but I didn't try to.

The patch comes with basic unit tests (for tag command not checkout command, as the fix is identical) and I tested the patch locally doing a release of a project onto a ssh based server. Both release:prepare and release:perform now work.

I don't give rights to Apache with regard to reusing the code.

Kidding...