ignore trailing slash for `guix gc -d' /gnu/store/...something/

[sleep_walker]

Package: guix
Version: 0.8.1

When I invoke `guix gc -d /path', success of the command depends whether /path 
ends with slash or not:

$ guix gc -d /gnu/store/mgqk0kv5jckrd7l5cwj68fp7cz2l4qqj-luajit-2.0.3/
guix gc: error: build failed: path 
`/gnu/store/mgqk0kv5jckrd7l5cwj68fp7cz2l4qqj-luajit-2.0.3/' is not in the Nix 
store

$ guix gc -d /gnu/store/mgqk0kv5jckrd7l5cwj68fp7cz2l4qqj-luajit-2.0.3
finding garbage collector roots...
cannot read potential root `/var/guix/manifests'
deleting `/gnu/store/73b760g2nr9syhznydgly8r7vl2k9z11-profile'
deleting `/gnu/store/mgqk0kv5jckrd7l5cwj68fp7cz2l4qqj-luajit-2.0.3'
deleting `/gnu/store/trash'
deleting unused links...
note: currently hard linking saves 247.60 MiB


* with the misleading error message it is usability bug
* it should be safe always try to remove slash from the end
* other commands may be affected too and the behaviour should be consistent for 
all commands