Question

How can we abort the merge if there is a merge conflict?

Answer 1

Since your pull was unsuccessful then HEAD (not HEAD^) is the last "valid" commit on your branch:

git reset --hard HEAD

The other piece you want is to let their changes over-ride your changes.

Older versions of git allowed you to use the "theirs" merge strategy:

git pull --strategy=theirs remote_branch

git fetch origin
git reset --hard origin