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.