args: treat --count --only-matching as --count-matches

Namely, when ripgrep is asked to count things and is also asked to print
every match on its own line, then we should just automatically count the
matches and not the lines. This is a departure from how GNU grep behaves,
but there is a compelling argument to be made that GNU grep's behavior
doesn't make a lot of sense.

Note that since this changes the behavior of combining two existing
flags, this is a breaking change.

tests/tests.rs

@@ -426,6 +426,13 @@ sherlock!(count_matches_inverted, "Sherlock", ".", |wd: WorkDir, mut cmd: Comman
     assert_eq!(lines, expected);
 });
 
+sherlock!(count_matches_via_only, "the", ".", |wd: WorkDir, mut cmd: Command| {
+    cmd.arg("--count").arg("--only-matching");
+    let lines: String = wd.stdout(&mut cmd);
+    let expected = "sherlock:4\n";
+    assert_eq!(lines, expected);
+});
+
 sherlock!(files_with_matches, "Sherlock", ".", |wd: WorkDir, mut cmd: Command| {
     cmd.arg("--files-with-matches");
     let lines: String = wd.stdout(&mut cmd);