git: Use git branch -f to reset branch pointers
git: Use git branch -f to reset branch pointers

Use `git branch -f branch position` to reset branch pointers instead of
using `git reset --keep`. This way is much faster and more robust since
it does not include checking out of the branch that is to be reset.

Change-Id: 0d662af344
Story-Id: SF-250

salsaflow/salsaflow,salsaflow/salsaflow