Fix prefix error triggered by Helm

See docstring in changes.

layers/+completion/spacemacs-ivy/packages.el

@@ -157,6 +157,27 @@ that directory."
   (use-package counsel
     :config
     (progn
+      (defun spacemacs/describe-mode ()
+        "Dummy wrapper to prevent an key binding error from helm.
+
+By default the emacs leader is M-m, turns out that Helm does this:
+
+   (cl-dolist (k (where-is-internal 'describe-mode global-map))
+        (define-key map k 'helm-help))
+
+after doing this:
+
+   (define-key map (kbd \"M-m\") 'helm-toggle-all-marks)
+
+So when Helm is loaded we get the error:
+
+   Key sequence M-m h d m starts with non-prefix key M-m
+
+To prevent this error we just wrap `describe-mode' to defeat the
+Helm hack."
+        (interactive)
+        (call-interactively 'describe-mode))
+
       (spacemacs/set-leader-keys
         dotspacemacs-command-key 'counsel-M-x
         ;; files
@@ -164,7 +185,7 @@ that directory."
         "fL"  'counsel-locate
         ;; help
         "hdf" 'counsel-describe-function
-        "hdm" 'describe-mode
+        "hdm" 'spacemacs/describe-mode
         "hdv" 'counsel-describe-variable
         ;; insert
         "iu"  'counsel-unicode-char