Git messages follows ChangeLog format, which you could apply by using
‘etc/snippets’ in case you are an Emacs user.  See
https://www.gnu.org/software/guix/manual/html_node/The-Perfect-Setup.html

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gnu: Add sbcl-stumpwm-pass.

* gnu/packages/wm.scm (sbcl-stumpwm-pass): New variable.
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Oleg.