Speed up pyenv prefix by not constructing advice text when it would be discarded (#3005)

by adding an internal flag

test/which.bats

@@ -155,3 +155,15 @@ exit
   PYENV_VERSION=3.4 run pyenv-which python
   assert_success "version=3.4.2"
 }
+
+@test "skip advice supresses error messages" {
+  create_executable "2.7" "python"
+  create_executable "3.3" "py.test"
+  create_executable "3.4" "py.test"
+
+  PYENV_VERSION=2.7 run pyenv-which py.test --skip-advice
+  assert_failure
+  assert_output <<OUT
+pyenv: py.test: command not found
+OUT
+}