Add a variable GIT_BRANCH which specifies a branch or tag to initially clone from.

For example, if the following is added to poudriere.conf:

GIT_URL=https://github.com/freenas/ports.git
GIT_BRANCH=9.2.1-BRANCH

and the following command is run:

poudriere -c -p freenasports -m git

Then poudriere will check out the ports by doing:

git clone -b 9.2.1-BRANCH https://github.com/freenas/ports.git

This is useful for the FreeNAS project, which maintains a
separate mirror of the FreeBSD ports repository on github,
with different branches and tags.