test-hgignore: conditionalize an illegal Windows filename

mac: fix percent-encoding of non-utf-8 characters (issue4999)

help: support loading sub-topics If a sub-topic/section is requested and the main topic corresponds to a topic with sub-topics, we now look for and return content for a sub-topic if found. With this patch, `hg help internals.X` now works. hgweb does not yet render sub-topics, however.