Better filetype detection for temp changes files

[jcronenberg]

Right now the tempfile when invoking the osc vc command has a naming scheme that results in something like package.changes.vctmp.slkqwo. This has the problem that editors then can't as easily detect that it is a .changes file. This PR changes the naming scheme so that it would be something like vctmp.slkqwo.package.changes

[mlschroe]

Thanks!