emacs: Add and use 'guix-concat-strings'.

* emacs/guix-utils.el (guix-concat-strings): New function.
* emacs/guix-pcomplete.el (guix-pcomplete-short-options): Use it.
* emacs/guix-base.el (guix-get-package-strings): Likewise.

emacs/guix-base.el

@@ -913,7 +913,7 @@ ENTRIES is a list of package entries to get info about packages."
                  (concat (guix-get-full-name entry)
                          (when outputs
                            (concat ":"
-                                   (mapconcat #'identity outputs ",")))
+                                   (guix-concat-strings outputs ",")))
                          (when location
                            (concat "\t(" location ")")))))))
          specs)))

emacs/guix-pcomplete.el

@@ -147,7 +147,7 @@ subcommands, actions, etc. for this guix COMMAND."
   "Return a string with available short options for guix COMMAND."
   guix-pcomplete-parse-short-option-regexp
   (lambda (list)
-    (mapconcat #'identity list "")))
+    (guix-concat-strings list "")))
 
 (guix-memoized-defun guix-pcomplete-all-packages ()
   "Return a list of all available Guix packages."

emacs/guix-utils.el

@@ -128,6 +128,22 @@ split it into several short lines."
       (fill-region (point-min) (point-max)))
     (buffer-string)))
 
+(defun guix-concat-strings (strings separator &optional location)
+  "Return new string by concatenating STRINGS with SEPARATOR.
+If LOCATION is a symbol `head', add another SEPARATOR to the
+beginning of the returned string; if `tail' - add SEPARATOR to
+the end of the string; if nil, do not add SEPARATOR; otherwise
+add both to the end and to the beginning."
+  (let ((str (mapconcat #'identity strings separator)))
+    (cond ((null location)
+           str)
+          ((eq location 'head)
+           (concat separator str))
+          ((eq location 'tail)
+           (concat str separator))
+          (t
+           (concat separator str separator)))))
+
 (defun guix-completing-read-multiple (prompt table &optional predicate
                                       require-match initial-input
                                       hist def inherit-input-method)