globset: fix repeated use of **

This fixes a bug where repeated use of ** didn't behave as it should. In
particular, each use of `**` added a new requirement directory depth
requirement. For example, something like `**/**/b` would match
`foo/bar/b`, but it wouldn't match `foo/b` even though it should. In
particular, `**` semantics demand "infinite" depth, so repeated uses of
`**` should just coalesce as if only one was given.

We do this coalescing in the parser. It's a little tricky because we
treat `**/a`, `a/**` and `a/**/b` as distinct tokens with their own
regex conversions. We also test the crap out of it.

Fixes #1174

tests/regression.rs

@@ -604,3 +604,12 @@ rgtest!(r1173, |dir: Dir, mut cmd: TestCommand| {
     dir.create("foo", "test");
     cmd.arg("test").assert_err();
 });
+
+// See: https://github.com/BurntSushi/ripgrep/issues/1174
+rgtest!(r1174, |dir: Dir, mut cmd: TestCommand| {
+    dir.create_dir(".git");
+    dir.create(".gitignore", "**/**/*");
+    dir.create_dir("a");
+    dir.create("a/foo", "test");
+    cmd.arg("test").assert_err();
+});