templatekw: add 'subrepos' keyword to show updated subrepositories 'subrepos' template keyword newly added by this patch shows updated subrepositories. For the compatibility with the list of subrepositories shown in the editor at commit: - 'subrepos' is empty, at revisions removing '.hgsub' itself - 'subrepos' is calculated between the revision and the first parent of it, at merge revisions To avoid silent regression, this patch also confirms "hg diff" of ".hgsubstate" and parents for each target revisions in the test.