Added --start-point/-s argument to payu clone to specify what commit/tag/branch name to start new branches from.

So there is still -B/--branch flag which initially checks out a branch during the git clone command. This was initially done (I think?) to avoid checking out a default branch first. For example, the access-om2-configs repository, the main branch is not a payu configuration.

The start point is used similarly to the payu checkout's positional argument, so

payu clone -b ${NEW_BRANCH_NAME} -s ${COMMIT_OR_TAG} ${REPOSITORY} ${DIR}

is equivalent to running

payu clone ${REPOSITORY} ${DIR}
payu checkout -b ${NEW_BRANCH_NAME} ${COMMIT_OR_TAG}

Closes #503