Merge branch 'rs/git-gui-use-modern-git-merge-syntax' into maint

The original command line syntax for "git merge", which was "git merge <msg> HEAD <parent>...", has been deprecated for quite some time, and "git gui" was the last in-tree user of the syntax. This is finally fixed, so that we can move forward with the deprecation. * rs/git-gui-use-modern-git-merge-syntax: git-gui: stop using deprecated merge syntax