dpkg-source/3.0 (quilt): improve reliability of check_patches_applied

dpkg-source/3.0 (quilt): improve reliability of check_patches_applied

* scripts/Dpkg/Source/Patch.pm (check_apply): New function to
verify if a patch will successfully apply on top of a given
directory.
* scripts/Dpkg/Source/Package/V3/quilt.pm (check_patches_applied):
Don't trust debian/patches/.dpkg-source-applied blindly. Get a
list of (supposedly unapplied) patches and verify if the first
patch applies or not. If yes, then apply the patch series,
otherwise do not (and assume that the patch series is already
applied).