Hi,
to switch to the stable branch
cd /usr/src && git checkout DragonFly_RELEASE_2_10
from that point on git pull in /usr/src or make src-update in /usr will keep you up to date on the stable branch.
For more info, have a look at /usr/src/{README,UPDATING} and section 7 of the man pages, esp. build(7).
Regards,
matthiasr