Merge branch 'sm/difftool'

Use of some uninitialized variables in "git difftool" has been
corrected.

* sm/difftool:
  builtin/difftool: intialize some hashmap variables

t/t7800-difftool.sh

@@ -666,6 +666,10 @@ run_dir_diff_test 'difftool --dir-diff syncs worktree without unstaged change' '
 	test_cmp expect file
 '
 
+run_dir_diff_test 'difftool --dir-diff with no diff' '
+	git difftool -d main main
+'
+
 write_script modify-file <<\EOF
 echo "new content" >file
 EOF