commitpkg: Sync changelog/install check with makepkg(8)

makepkg(8) currently uses a smarter method to extract all changelog and
install files from a PKGBUILD. Sync commitpkg to use the same code (with
small modifications). This also adds support for changelog/install files
that contain a whitespace.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>