feat: update mergetools to add mergiraf and remove sourcetree

gimmes/tools/mergetool

@@ -2,7 +2,9 @@
 set -o errexit -o pipefail
 
 if [[ "$OS" = "Darwin" ]]; then
-	gimme sourcetree
+	gimme \
+        mergiraf \
+        --
 else
 	gimme meld
 fi