Don't print empty lines in single threaded mode.

Fixes #99.

tests/tests.rs

@@ -54,6 +54,13 @@ fn path(unix: &str) -> String {
     }
 }
 
+fn sort_lines(lines: &str) -> String {
+    let mut lines: Vec<String> =
+        lines.trim().lines().map(|s| s.to_owned()).collect();
+    lines.sort();
+    format!("{}\n", lines.join("\n"))
+}
+
 sherlock!(single_file, |wd: WorkDir, mut cmd| {
     let lines: String = wd.stdout(&mut cmd);
     let expected = "\
@@ -716,6 +723,18 @@ clean!(regression_93, r"(\d{1,3}\.){3}\d{1,3}", ".",
     assert_eq!(lines, "foo:192.168.1.1\n");
 });
 
+// See: https://github.com/BurntSushi/ripgrep/issues/99
+clean!(regression_99, "test", ".",
+|wd: WorkDir, mut cmd: Command| {
+    wd.create("foo1", "test");
+    wd.create("foo2", "zzz");
+    wd.create("bar", "test");
+    cmd.arg("-j1").arg("--heading");
+
+    let lines: String = wd.stdout(&mut cmd);
+    assert_eq!(sort_lines(&lines), sort_lines("bar\ntest\n\nfoo1\ntest\n"));
+});
+
 // See: https://github.com/BurntSushi/ripgrep/issues/105
 clean!(regression_105_part1, "test", ".", |wd: WorkDir, mut cmd: Command| {
     wd.create("foo", "zztest");