List diaries only if output is not a pipe

Beforehand, this command:

    imag diary -d foo list | cat

listed each entry twice because the reporting feature of libimagrt
forwarded the list to stdout and the normal output was written to
stderr.

With this patch, we get the output only once in this case.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

bin/domain/imag-diary/src/list.rs

@@ -57,7 +57,9 @@ pub fn list(rt: &Runtime) {
         .for_each(|id| {
             let _ = rt.report_touched(&id).unwrap_or_exit();
 
+            if !rt.output_is_pipe() {
                 writeln!(rt.stdout(), "{}", id).to_exit_code().unwrap_or_exit()
+            }
         });
 }