Based on a patch by Nikita Karetnikov <nikita@karetnikov.org>.
* guix-package.in (profile-regexp): New procedure.
(latest-profile-number): Remove `%profile-rx', and use
`profile-regexp' instead.
(profile-number, roll-back): New procedure.
(show-help): Add `--roll-back'.
(%options): Likewise.
(guix-package)[process-actions]: First check whether `roll-back?' is
among OPTS, and call `roll-back' if it is, followed by a recursive
call to `process-actions'. Emit the "nothing to be done" message only
when INSTALL or REMOVE is non-empty.
* tests/guix-package.sh (readlink_base): New function.
Add tests for `--roll-back'.
* doc/guix.texi (Invoking guix-package): Document `--roll-back'.
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