git merge/rebase ref -P ref

The inverted meaning of {ours,theirs} for rebase could be very
confusing to some, especially to new uses, for me every time I
merge/rebase I need to think about it to make sure I've made it right.

What about making it more intuitive?

We can and a new option (like '-P') for people to specify the
preferred branch/ref by it's name.

E.g. assume I have two branches, namely 'dev' and 'exp', and I prefer
the changeset on the 'dev' branch when I merge or rebase, so that I
can do it with

# using merge
git checkout dev
git merge exp -P dev

OR

# using rebase
git checkout exp
git rebase dev -P dev