Modify git-merge-forward.sh so it can create test merge branches

We should modify git-merge-forward.sh so it takes an optional branch base name.

If the branch base name is set to bug1234, the script should:

1. Create bug1234_029 from the local maint-0.2.9
2. Create bug1234_035 from the local maint-0.3.5, and merge bug1234_029 into it
3. Repeat step 2 with each supported maint branch, and master

We might also want to modify git-push-all.sh to take a branch base name, and push the branches created by git-merge-forward.sh.