git mergetool ignores configured command line

To configure diffuse as merge tool I set mergetool configuration using
following commands:

git config --global merge.tool diffuse
git config --global mergetool.diffuse.cmd '/usr/bin/diffuse "$LOCAL"
"$MERGED" "$REMOTE"'

Then, when I invoke mergetool during merge, I get diffuse with four
panes open as by default, instead of three specified in command line
setting.
Is this an expected behavior, and how can I force mergetool to use
provided command line?