nobacktick test: Fix sed error on Mac OS, also ignore entire commented lines.

tests/t0100-code-nobacktick.sh

@@ -5,11 +5,13 @@ test_description='no old-style backtick command substitution
 This test checks the todo.sh script itself for occurrences
 of old-style backtick command substitution, which should be
 replaced with $(...).
+On failure, it will print each offending line number and line.
 '
 . ./test-lib.sh
 
 test_todo_session 'no old-style backtick command substitution' <<EOF
->>> sed -n -e 's/[ \t]#.*//' -e '/\d96/{=;p}' "$(which todo.sh)"
+>>> sed -n -e 's/\(^\|[ \t]\)#.*//' -e '/\d96/{' -e '=;p' -e '}' "$(which todo.sh)"
 EOF
+# Note: Must encode backtick as \d96 to avoid error in the test framework.
 
 test_done