fix: consecutive leading **/ should match like a single **/

[spokodev]

A pattern with consecutive leading globstars such as **/**/foo fails to match foo at the repository root, while git check-ignore matches it.

ignore().add('**/**/foo').ignores('foo')      // false, should be true
ignore().add('**/**/foo').ignores('a/foo')    // true (correct)

$ printf '**/**/foo\n' > .gitignore
$ git check-ignore --no-index foo
foo

Only the root case is wrong; a/foo and a/b/foo already match. Git treats a run of leading **/ the same as a single **/ ("match in all directories", including zero), so **/**/foo is equivalent to **/foo.

Cause: the leading-globstar replacer only consumes the first **/. The remaining **/ is then handled as an ordinary **, which forces at least one intermediate directory and drops the root match.

Fix: let the replacer consume one or more consecutive leading **/ runs. **/**/foo now compiles to the same regex as **/foo.

Adds a regression test to test/fixtures/cases.js (exercised by .ignores(), .filter(), .checkIgnore() and the git check-ignore fixture oracle).