gnu: git-flow: Use 'modify-phases'.

* gnu/packages/version-control.scm (git-flow)[arguments]: Use
'modify-phases'.

gnu/packages/version-control.scm

@@ -490,8 +490,9 @@ will work.")
      '(#:tests? #f                    ; no tests
        #:make-flags (list (string-append "prefix="
                                          (assoc-ref %outputs "out")))
-       #:phases (alist-cons-after
-                 'unpack 'reset-shFlags-link
+       #:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'reset-shFlags-link
            (lambda* (#:key inputs #:allow-other-keys)
              ;; The link points to a file in the shFlags submodule.
              ;; Redirect it to point to our system shFlags.
@@ -499,10 +500,9 @@ will work.")
                (begin
                  (delete-file "gitflow-shFlags")
                  (symlink (string-append shflags "/src/shflags")
-                                "gitflow-shFlags"))))
-                 (alist-delete
-                  'configure
-                  (alist-delete 'build %standard-phases)))))
+                          "gitflow-shFlags")))))
+         (delete 'configure)
+         (delete 'build))))
     (home-page "http://nvie.com/posts/a-successful-git-branching-model/")
     (synopsis "Git extensions for Vincent Driessen's branching model")
     (description