test-share: handle sed/GNU-sed discrepancies

Make {file_copies} usable as a --template key Before this, to display file copies in templates, you had to write something like {file_copies%filecopy}. For some reason, the {file_copy} subtemplate was used by default but not defined by default in changeset_templater, while styles were already using it. Here we define {file_copy} in changeset_templater, and change the templater to handle formatting strings like {file_copies%filecopy} with already expanded keys (in this case {file_copies}), for backward compatibility.

templatekw: change {file_copies} behaviour, add {file_copies_switch} {file_copies} template now displays file copies with or without the --copies switch being set. A new {file_copies_switch} template implements the former behaviour.