Git: Discard all changes on a diverged local branch

Question

I have a local topic branch that's tracking a remote branch. For the sake of argument, say the commit histories look like this:

A--B--C--O1--O2--O3 (origin/phobos)
       \
         L1--L2--L3 (phobos)

Having looked at the relative commit histories, I now want to discard all the changes to the local phobos branch and get it back to being a direct copy of origin/phobos, so that the local history looks like this:

A--B--C--O1--O2--O3 (phobos origin/phobos)

I really don't want the local changes to the phobos branch, and I really don't want any merges to show up in the origin repository afterwards. (So, just merging isn't what I have in mind.)

This seems like it should be really easy, but my google-fu has failed me. How do I do this?

Answer 1

Delete the branch, then re-create it:

$ git branch -D phobos
$ git checkout --track -b phobos origin/phobos

Answer 2

git checkout phobos
git reset --hard origin/phobos

This tells Git to reset the head of phobos to the same commit as origin/phobos, and to update the working tree to match.