** never matches relative paths

[BurntSushi]

Originally reported by @okdana in #1093 (comment)

What version of ripgrep are you using?

master @ b48bbf5

How did you install ripgrep?

Compiled.

What operating system are you using ripgrep on?

Linux.

Describe your question, feature request, or bug.

The ** pattern does not work in a .gitignore file.

If this is a bug, what are the steps to reproduce the behavior?

$ mkdir /tmp/rgbug
$ cd /tmp/rgbug
$ git init
$ echo '**' > .gitignore
$ echo test > foo
$ rg test
foo
1:test

If this is a bug, what is the actual behavior?

foo is shown as a search result, but it should be ignored.

If this is a bug, what is the expected behavior?

foo should have been ignored by the ** pattern.