Fix substitution for hiding contexts/projects

When using the -+ or -@ option to hide project or context names in list
output, the color reset sequence was being deleted if the project/context
was at the end of the line.

For example, with the following tasks:

    (B) with priority, ending in a +project
    task that shouldn't be highlighted

Running 'todo.sh -+ list' would hide the "+project" from the first task,
but would also remove the color reset sequence ^[[0m at the end.  The
color from the prioritized task would continue to the next lines.

This commit fixes the problem and adds a test to verify the fix.

tests/t1330-ls-highlighting.sh

@@ -148,4 +148,22 @@ TODO: 4 marked as done.
 TODO: 5 of 5 tasks shown
 EOF
 
+# check highlighting with hidden contexts/projects
+#
+cat > todo.txt <<EOF
+(A) +project at the beginning, with priority
+(B) with priority, ending in a +project
+(C) @context at the beginning, with priority
+(Z) with priority, ending in a @context
+EOF
+test_todo_session 'highlighting with hidden contexts/projects' <<EOF
+>>> todo.sh -+ -@ list
+1 (A) at the beginning, with priority
+2 (B) with priority, ending in a
+3 (C) at the beginning, with priority
+4 (Z) with priority, ending in a
+--
+TODO: 4 of 4 tasks shown
+EOF
+
 test_done