walk: no longer ignore revlogs of files starting with `undo.` (issue6542) Changeset 0b569c75d180 introduced new code in store.walk to filter out undo files left behind by the transaction. However doing so is also filtering out legitimate revlog file starting with `undo.` So this changeset is mostly rolling back that change and adding tests tests to catch this kind of error in the future. As a result we the transaction undo files a considered again by various code (in practice mostly persistent nodemap related). We either live with it (low inconvenient) or explicitly work around it for now. This should be good enough to no longer block the 5.9rc release with this issue. We shall build something cleaner within the 6.0 cycle. Differential Revision: https://phab.mercurial-scm.org/D11201