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.