regtest: add regression testing tool
emit empty an empty item to reset list attributes

Given something like the following PKGBUILD:

  pkgbase=foo
  pkgname=(foo bar)
  arch=(any)
  pkgver=1
  pkgrel=1
  depends=(quite some depends)

  package_foo() {
    pkgdesc="all the depends"
  }

  package_bar() {
    pkgdesc="none of the depends"
    depends=()
  }

makepkg will treat this as the descriptions state, but our AURINFO
format would ignore the empty declaration, meaning that the subpkg
incorrectly inherit the global value. There's at least 2 ways to solve
this:

1) Do the attribute merging for lists in the subpkgs themselves.
2) Emit an empty list, require parsers to handle this as a reset.

This commit implements the second option. Added bonus: the refactoring
inadvertantly fixes another, unrelated, inheritance bug and provides a
marked improvement in accuracy.
give meaningful names to vars
add modelines for python scripts