Rubymine provides a visual merge conflict resolution tool

RubyMine provides a visual tool for resolving merge conflicts locally.

Git > Resolve Conflicts

in the context menu to open RubyMine's merge conflict tool.

Left pane: local copy (read-only)
Right pane: checked in version from repository (read-only)
Central pane: base revision from which both conflicting versions are derived

You can also use a similar pane view to compare to files.
Mark two files and press Ctrl + D to compare.