Why won't Git override my merge.ff setting from the command line?

Question

I've set merge.ff to only in my local Git config:

$ git config --list | grep merge
merge.ff=only

This successfully prevents Git from performing a non-fast-forward merge, as expected.

However, when I want to explicitly allow such a merge and try to override that setting from the command line (either directly or as per this answer), I can't:

$ git merge --no-ff other-branch
fatal: You cannot combine --no-ff with --ff-only.
$ git -c merge.ff=false merge other-branch
fatal: You cannot combine --no-ff with --ff-only.
$ git -c merge.ff=true merge other-branch
fatal: Not possible to fast-forward, aborting.

What am I missing?

Answer 1

I probably found the solution of that problem. According to the release-notes of git 1.8.4 the cli was not working correctly:

* 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.