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.

src/output.rs

@@ -85,10 +85,10 @@ impl OutputType {
 
         match pagerflags.split_first() {
             Some((pager_name, args)) => {
-                let mut pager_path = PathBuf::from(pager_name);
+                let pager_path = PathBuf::from(pager_name);
 
                 if pager_path.file_stem() == Some(&OsString::from("bat")) {
-                    pager_path = PathBuf::from("less");
+                    return Err(ErrorKind::InvalidPagerValueBat.into());
                 }
 
                 let is_less = pager_path.file_stem() == Some(&OsString::from("less"));