* guix/scripts/package.scm (transaction-upgrade-entry)[upgrade]: Add
'transform' parameter. Pass PKG through it. Use
'manifest-entry-with-transformations'.
Call 'options->transformation' to get the transformation procedure.
* tests/guix-package.sh: Add 'guix package -u' test.
* tests/packages.scm ("transaction-upgrade-entry, transformation options preserved"):
New test.
* doc/guix.texi (Invoking guix package): Mention that transformations
are preserved across upgrades.
(Package Transformation Options): Likewise.
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