OUT/Repo: Add code to determine if a tracking branch already exists

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
OUT/Repo: implement branch dropping

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>
init-upstream-tracking: add force argument to drop existing tracking branch if neccesary

Signed-off-by: Gregory M. Turner <gmt@be-evil.net>