rebase: extract _assignworkingcopy
Slightly cleaner, and makes it easier to hook this in an extension.
Differential Revision: https://phab.mercurial-scm.org/D1617
changeset: {node}
revision: {rev}
user: {author}
date: {date|rfc822date}
summary: {desc}
{branches%branchname}{tags%tagname}{bookmarks%bookmarkname}