mu4e: rework mu4e--file-name-concat into mu4e-join-paths

We also need to handle slashes at the *start*.

mu4e/mu4e-helpers.el

@@ -460,20 +460,16 @@ Or go to the top level if there is none."
 
 
 ;;; Misc
-(defun mu4e--file-name-concat (directory &rest components)
+(defun mu4e-join-paths (directory &rest components)
   "Append COMPONENTS to DIRECTORY and return the resulting string.
-Elements in COMPONENTS must be a string or nil. DIRECTORY or the
-non-final elements in COMPONENTS may or may not end with a slash
--- if they don’t end with a slash, a slash will be inserted
-before concatenating.
 
-Compatibility function for Emacs 28's `file-name-concat'."
-  (mapconcat
-   (lambda (part)
-     (if (string= (substring part -1) "/")
-         (substring part 0 -1)
-       part))
-   (cons directory components) "/"))
+This is mu4e's version of Emacs 28's `file-name-concat' with the
+difference it also handles slashes at the beginning of
+COMPONENTS."
+  (replace-regexp-in-string
+   "//+" "/"
+   (mapconcat (lambda (part) (if (stringp part) part ""))
+              (cons directory components) "/")))
 
 (provide 'mu4e-helpers)
 ;;; mu4e-helpers.el ends here