Uniformly use $TODO_SH to refer to script.

Set $TODO_SH to just be $(basename "$0") at the beginning
of the script and use it in all usage messages.

This happens to simplify testing because it allows us to refer to
the script as "todo.sh" in all output.  For general use, if the
user has aliased to file to something else, the usage messages will
still reflect that alias.

Signed-off-by: Emil Sit <sit@emilsit.net>

tests/t9999-testsuite_example.sh

@@ -92,7 +92,7 @@ TODO: Report file updated.
 2009-02-13-04:40:00 5 1
 
 >>> todo.sh append g a
-usage: $HOME/bin/todo.sh append ITEM# "TEXT TO APPEND"
+usage: todo.sh append ITEM# "TEXT TO APPEND"
 === 1
 
 >>> todo.sh append 2 and think