IDEA-99129 diff: remember 'external merge tool' settings

(cherry picked from commit 365c3b8ae08d7503100e30452713d651a960684c)
diff: enable/disable 'merge parameters' text field depending on 'Merge tool enabled' checkbox

(cherry picked from commit 5eec29144dee9251ffd8e4bdc8a7c5e35afd08f7)