Fixes <https://issues.guix.gnu.org/49697>.
Reported by Philippe Swartvagher <philippe.swartvagher@inria.fr>.
* guix/transformations.scm (patched-source): New procedure.
(transform-package-patches)[package-with-extra-patches]: Use it
when (package-source p) is not an origin.
* tests/transformations.scm ("options->transformation, with-commit +
with-patch"): New test.