Fix index file's mtime, which can differ from TIMESTAMP.

Fix index file's mtime, which can differ from TIMESTAMP.

This enables Portage to reliably query for remote indices with
HTTP-header If-Modified-Since.

Without this patch mtime is greater than TIMESTAMP for large
indices and slow storages - because writing a large file takes
time. If the difference spans a second (TIMESTAMP 08:00:00, mtime
08:00:01), then Portage will always fetch the remote index because
it will appear being modified (mtime is used there) after the copy
has been made (local copy's TIMESTAMP is used here).