Before this commit, the root directory would be removed which leaves a
trailing file separator character in the beginning. This would then be trimmed
by using strings.TrimPrefix. The problem here is that it uses a hardcoded
file separator character which won't work on Windows because
Windows uses a backslash instead of a forward slash.

Using filepath.Rel to get the directory relative to the root directory
will not leave a separator character in the beginning.

It's also important to use filepath.ToSlash which will translate every
file separator character to the forward slash.

This commit makes discovery work on Windows which it didn't before.

Rel returns . when the directory is the same as the base directory.
We can easily translate that to the empty string which is what we're
looking for.

filepath.Dir will return the same result when you're calling it on the
root directory of your filesystem. Checking when this happens is
required so that you break out of your loop.

This code used to loop endlessly when called outside of a git repo.
This commit fixes that.