Fix git:branch bug when there is more than one branch with equal SHA-ID

This fixes badmatch error when we have more than one branch with equal sha-id. In such case, you tried to match one-element list ([B]) with list of branches (["branchA", "branchB"]). Now we just return list of branches, separated with "; ", ex. "branchA; branchB"
Use string:join/2 instead