Change /patch to return base64 encoded

This is easier to work with on the command line. You can now do
something trivial like:

  curl http://localhost:8080/changes/130/revisions/2/patch | base64 -d | git am

While we are here fix up the header output to better match what
Git does when formatting a commit as an email header for `git am`
to consume.

Change-Id: I0a01360788d9c528a78314b750405533501c5fff
2 files changed