Try merging through right click on another branch. There isn't an option to just merge without committing.

The option is present in Merge dialog opened from the toolbar but it doesn't really function. If I uncheck all the checkboxes in that dialog I expect to end up with merged changes in the working tree (git merge --no-commit --no-ff <branch>). Instead it commits/fast forwards regardless of the checkboxes.

On Mac the option is present in both dialogs - and it works.