amend: add a flag `-n/--note` to store note with amend This patch utilises the functionality added in previous patches and adds a flag to amend command in hgext/amend to add a note to the amend. Since the note is stored in the obsmarker metadata, this will only be useful when obsmarker creation is enabled, otherwise this is no-op. Not adding releasenotes part as we yet don't have a functionality in core to show the note. Differential Revision: https://phab.mercurial-scm.org/D1095