magit-prompt-with-default: new function

Split `magit-prompt-with-default' from `magit-builtin-completing-read'
so that it could also be used by other `completing-read' replacements
and wrappers.

magit.el

@@ -3749,12 +3749,9 @@ Read `completing-read' documentation for the meaning of the argument."
 (defun magit-builtin-completing-read
   (prompt choices &optional predicate require-match initial-input hist def)
   "Magit wrapper for standard `completing-read' function."
-  (let ((reply (completing-read
-                (if (and def (> (length prompt) 2)
-                         (string-equal ": " (substring prompt -2)))
-                    (format "%s (default %s): " (substring prompt 0 -2) def)
-                  prompt)
-                choices predicate require-match initial-input hist def)))
+  (let ((reply (completing-read (magit-prompt-with-default prompt def)
+                                choices predicate require-match
+                                initial-input hist def)))
     (if (string= reply "")
         (if require-match
             (user-error "Nothing selected")
@@ -3786,6 +3783,12 @@ Read `completing-read' documentation for the meaning of the argument."
                                          choices)))))
     (iswitchb-read-buffer prompt (or initial-input def) require-match)))
 
+(defun magit-prompt-with-default (prompt def)
+  (if (and def (> (length prompt) 2)
+           (string-equal ": " (substring prompt -2)))
+      (format "%s (default %s): " (substring prompt 0 -2) def)
+    prompt))
+
 ;;;;; Revision Completion
 
 (defvar magit-read-rev-history nil