scmutil: add capability to cleanupnodes to take obsmarker metadata This patch adds a metadata argument to cleanupnodes() which will be dict and can be passed to obsmarker.createmarkers() and can be stored on the obsmarker. In cases when obsolescence is not enabled, the metadata argument is useless. This is a step towards storing a note in amend. Differential Revision: https://phab.mercurial-scm.org/D1093