svntogit: Set SHELL=/bin/bash for git pull to work

The default login shell for the svntogit user (/sbin/nologin) breaks
the Match Exec directives in /srv/svntogit/.ssh/config and prohibits
Git from using the correct SSH key.

While we're at it, add --set-upstream to the git pull command so the
task is more likely to accomplish its intended purpose.