Merge branch 'mv/merge-ff-tristate'

The configuration variable "merge.ff" was cleary a tri-state to choose one from "favor fast-forward when possible", "always create a merge even when the history could fast-forward" and "do not create any merge, only update when the history fast-forwards", but the command line parser did not implement the usual convention of "last one wins, and command line overrides the configuration" correctly. * mv/merge-ff-tristate: merge: handle --ff/--no-ff/--ff-only as a tri-state option