If you have a repository in Git, made a branch say new_branch and did some changes in both master and the new_branch.
After many commits when you are in a need to convert the new_branch to master and to discard the commits done on master, try with the following set of commands.
git checkout new_branch
git merge --strategy=ours master # it just records a commit, but retains the contents of new_branch
git checkout master
git merge new_branch # fast-forward master up to the merge
Pradeep's Ruby on Rails Blog 