repo: Make summary and signature mtime match

repo: Make summary and signature mtime match

HTTP servers derive Last-Modified from the modification time of the
file. When used in combination with a Cache-Control max-age value,
having the modification times match means that caches will consider them
expired at the same time. This helps make it more likely that clients
won't receive a cached summary and fresh signature or vice versa.

This makes more sense to do now that the summary and signature are
created in a temporary directory and renamed into place. In the old days
where they were created directly in the repo root, it would be strange
to change the summary mtime when it wasn't actually modified.