Add shebang line and check for Bash.

The shebang is ignored when sourcing the script (but still helps many text editors auto-detect the file type), and will cause an error when the script is mistakenly executed.
The Bash check allows to have this called from a generic place (e.g. .profile), and do no harm when under a different shell.

todo_completion

@@ -1,3 +1,6 @@
+#!/bin/bash source-this-script
+[ "$BASH_VERSION" ] || return
+
 _todo()
 {
     local cur prev opts