devtools: clean-up of commitpkg

- replaces backticks by $()
- removes support for package names without the arch in them (all our
repos now uses the arch in the package name)
- removes support for the obsolete and dead [unstable] repo

Patch-by: Eric Bélanger <eric@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>