Use an internal variable for concat to allow template resolution (#66)

Avoids redefining `concat` in el-patch-template.

Author: haji-ali

el-patch-template.el

@@ -523,12 +523,11 @@ Similar to `el-patch--resolve' with a special treatment for
 `el-patch-concat'. Specifically, if the arguments of
 `el-patch-concat' have `...' in them, it is not resolved but
 changed to `el-patch-template--concat'."
-  (cl-letf* ((old-concat (symbol-function 'concat))
-             ((symbol-function 'concat)
-              (lambda (&rest args)
-                (if (cl-some (lambda (x) (equal x '...)) args)
-                    (cons 'el-patch-template--concat args)
-                  (apply old-concat args)))))
+  (let ((el-patch--concat-function
+         (lambda (args)
+           (if (cl-some (lambda (x) (equal x '...)) args)
+               (cons 'el-patch-template--concat args)
+             (apply #'concat args)))))
     (el-patch--resolve forms nil)))
 
 ;; Stolen from el-patch--select

el-patch.el

@@ -197,6 +197,9 @@ when a patch renames a symbol.")
 (defvar el-patch--not-present (make-symbol "el-patch--not-present")
   "Value used as a default argument to `gethash'.")
 
+(defvar el-patch--concat-function #'concat
+  "Function to concatenate strings when resolving patches.")
+
 ;;;; Resolving patches
 
 (defmacro el-patch--with-puthash (table kvs &rest body)
@@ -346,7 +349,8 @@ their bindings."
            (when (<= (length form) 1)
              (error "Not enough arguments (%d) for `el-patch-concat'"
                     (1- (length form))))
-           (list (apply #'concat (cl-mapcan resolve (cdr form)))))
+           (list (apply el-patch--concat-function
+                        (cl-mapcan resolve (cdr form)))))
           (_
            (let ((car-forms (funcall resolve (car form)))
                  (cdr-forms (funcall resolve (cdr form))))