Set the Last-Modified header of the Download branches files to that of the last push (BB-7137)

The automatically generated zip,gz, and bz2 files of branches is really handy, but it would be great if the Last-Modified timestamp in the header was set to the timestamp of the last push (the last time the branch was modified).

The current timestamp of the Last-Modified header is the time the request for the archive was made.

We want to use the header to check if we need to download a new version. We use this to automatically update an apps content while in development.