Includes don't take into account inner paths

[nlopes]

If an include includes a file at dir/a.adoc, we read that file correctly. But if a.adoc has an include directive for b.adoc, we can't find b.adoc. That's either because we're at the parent directory and we're not adding dir/ or because we're in dir/ and trying to load from the parent directory.

[nlopes]

Fixed in 3c25552. Nested includes now resolve paths relative to their parent file's directory.