Fixes <http://bugs.gnu.org/19978>.
Reported by taylanbayirli@gmail.com (Taylan Ulrich Bayırlı/Kammer).
* guix/scripts/package.scm (delete-matching-generations): Warn when
CURRENT is in NUMBERS, and always remove it before calling
'delete-generations'.
* tests/guix-package.sh: Add --switch-generation=2 invocation before
--delete-generations=3 invocation.
Add --delete-generations=1.. test case.