To support git tags we need to take different semantic actions (as
checking-out a git tag leaves you in a detached-HEAD state where
git-{pull,merge,rebase} don't apply). To prepare for having different
methods for these different semantics I'm factoring out the git-remote
setup into a separate function, and removing the needless support for
lists of possibilities. Nowadays kdesrc-build only cares about origin,
though if you want to bring your own remote that's fine, kdesrc-build
will find it.