Limit line width in git commit messages to 72 characters

This is per https://cbea.ms/git-commit/#wrap-72 and makes the line width
in git commit messages independent of the global fill-column setting.

init.el

@@ -1304,7 +1304,10 @@
                                                 "Suggested-by"
                                                 "Reported-by"
                                                 "Tested-by"
-                                                "Reviewed-by")))
+                                                "Reviewed-by"))
+  :config (add-hook 'git-commit-mode-hook
+                    #'(lambda ()
+                        (setq fill-column 72))))
 
 (use-package highlight-indentation
   :commands highlight-indentation-mode)