ignore: fix handling of **

When deciding whether to add the `**/` prefix or not, we should choose
not to add it if the pattern is simply a bare `**`. Previously, we were
only not adding it if it was `**/`, which is correct, but we also need
to do it for `**` since `**` can already match anywhere.

There's likely a more principled solution to this, but this works for
now.

Fixes #1173

tests/regression.rs

@@ -596,3 +596,11 @@ rgtest!(r1164, |dir: Dir, mut cmd: TestCommand| {
         cmd.arg("--no-ignore-file-case-insensitive").stdout()
     );
 });
+
+// See: https://github.com/BurntSushi/ripgrep/issues/1173
+rgtest!(r1173, |dir: Dir, mut cmd: TestCommand| {
+    dir.create_dir(".git");
+    dir.create(".gitignore", "**");
+    dir.create("foo", "test");
+    cmd.arg("test").assert_err();
+});