The interactive prompt doesn't support src/ as a base path

[DrewAPicture]

In e3013dbfa2bd6b64d7b45c5f34f5b5123acaf364, you added the ability to interact with the prompt in the case where a file couldn't be found for patching. And this fixed #21.

However, in attempting to apply a patch today, I noticed that when interacting with the prompt, it would not allow me to specify the file path beginning with src/, only wp-*, in this case wp-includes/.

Seems like we'd want to cover the src/ case in the instance where files might fall in the root directory, not to mention, this is packaged with develop, so I'd expect it to work with src/ as a base path.

To reproduce, try applying post-thumbnail-comment-patch.diff via 31351 from the root directory, specifying the path src/wp-includes/post-thumbnail-template.php