Throws an error when bat is being user as pager.

As mentioned on #1334 `bat` should not be used as a value for `pager`,
this change checks both the balue of `bat` provided as a parameter or
as an environment variable.

tests/integration_tests.rs

@@ -405,6 +405,16 @@ fn pager_disable() {
         .stdout(predicate::eq("hello world\n").normalize());
 }
 
+#[test]
+fn pager_value_bat() {
+    bat()
+        .arg("--pager=bat")
+        .arg("--paging=always")
+        .arg("test.txt")
+        .assert()
+        .failure();
+}
+
 #[test]
 fn alias_pager_disable() {
     bat()