cmdutil: expand filename format string by templater (BC) This is BC because '{}' could be a valid filename before, but I believe good programmers wouldn't use such catastrophic output filenames. On the other hand, '\' has to be escaped since it is a directory separator on Windows. Thanks to Matt Harbison for spotting this weird issue. This patch also adds cmdutil.rendertemplate(ctx, tmpl, props) as a simpler way of expanding template against single changeset. .. bc:: '{' in output filename passed to archive/cat/export is taken as a start of a template expression.