compat: use cp -R in `test-bundle-phase-internal.t` (issue6835)

This follow POSIX and make things compatible with NetBSD.

tests/test-bundle-phase-internal.t

@@ -99,7 +99,7 b' backup bundle from strip'
 strip an ancestors of the internal changeset
 --------------------------------------------
 
-  $ cp -ar reference-repo strip-ancestor
+  $ cp -aR reference-repo strip-ancestor
   $ cd strip-ancestor
 
 The internal change is stripped, yet it should be skipped from the backup bundle.
@@ -154,7 +154,7 b' Shelve should still work'
 strip an unrelated changeset with a lower revnum
 ------------------------------------------------
 
-  $ cp -ar reference-repo strip-unrelated
+  $ cp -aR reference-repo strip-unrelated
   $ cd strip-unrelated
 
 The internal change is not directly stripped, but it is affected by the strip
@@ -200,7 +200,7 b' Shelve should still work'
 explicitly strip the internal changeset
 ---------------------------------------
 
-  $ cp -ar reference-repo strip-explicit
+  $ cp -aR reference-repo strip-explicit
   $ cd strip-explicit
 
 The internal change is directly selected for stripping.