fix creating branches.

fixes nirvdrum/svn2git/issues/142

Error output:

Running command: git branch --track "trunk@6615" "remotes/svn/trunk@6615"
fatal: Cannot setup tracking information; starting point 'remotes/svn/trunk@6615' is not a branch.
command failed:
2>&1 git branch --track "trunk@6615" "remotes/svn/trunk@6615"