Replace backticks in config with $(...)

This applies e24777fa2c to todo.cfg, too, and adds a verification to the existing test.

Note: Because todo.cfg is preprocessed by test-lib, we use the original config from the working copy, not the copy inside the test fixture.

tests/t0100-code-nobacktick.sh

@@ -16,6 +16,8 @@ backtick_check()
 
 test_todo_session 'no old-style backtick command substitution' <<EOF
 >>> backtick_check bin/todo.sh
+
+>>> backtick_check ../../todo.cfg
 EOF
 
 test_done