upload: add config setting for upload notifications

This allows people to set default e-mail notifications via gitconfig.

Bug: https://crbug.com/gerrit/12451
Change-Id: Ic04ea3b7df0c5603c491961112c5be8cabb9dddd
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259014


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>