Fix ignores when searching subdirectories

[sengaya]

When searching subdirectories the path was not correctly build and
included duplicate parts. This fix will remove the duplicate part if
possible. It also contains a small optimization that will skip the
second for loop if there is already a match.

Fixes #1757

[BurntSushi]

Nice, thank you! This fixes quite a gnarly bug that I've seen a lot of folks run into for a while. Thank you.

I ended up fixing two problems with this patch though:

1. First, the optimization does not look correct to me. The issue is that, for some ignore files, the existence of any rule regardless of where it is in the directory tree will override any other rule from a different type of ignore file. For example, .rgignore has higher precedent than .ignore. So you need to let all of the rules be visited. In practice, I doubt this optimization matters too much anyway.
2. This used Path::strip_prefix instead of ripgrep's own strip_prefix routine which does things a little differently. This meant that giving ripgrep a path like .////path would still trigger the bug. But using ripgrep's strip_prefix routine avoids this issue.

Overall this still feels Not Right to me, but it fixes a bug and doesn't seem to cause any other regressions. Hopefully we can straighten out this logic in a better way in the future when I overhaul the ignore crate.