Add testing of todo_completion.

Before adding any more features to todo_completion, I feel like I need test coverage, so this is a first stab at testing the completion results, via a new test function test_todo_completion.
Some basic tests showcase the capabilities.

Note: test-lib.sh now uses arrays, therefore all tests must use /bin/bash, not /bin/sh to avoid errors when sourcing test-lib. For consistency with todo.sh, we should have used Bash everywhere, anyway. Also note that t2000-multiline.sh needs some more quoting to avoid "Bash: ambiguous redirect" errors.

tests/README

@@ -125,10 +125,10 @@ Writing Tests
 -------------
 
 The test script is written as a shell script.  It should start
-with the standard "#!/bin/sh" with copyright notices, and an
+with the standard "#!/bin/bash" with copyright notices, and an
 assignment to variable 'test_description', like this:
 
-	#!/bin/sh
+	#!/bin/bash
 	#
 	# Copyright (c) 2005 Junio C Hamano
 	#
@@ -186,6 +186,17 @@ library for your script to use.
    expected output.  (See below for how to generate transcripts
    easily.)
 
+ - test_todo_completion <message> <cmdline> <completions>
+
+   This takes three strings as parameter.  Based on <cmdline>,
+   the todo_completion script is triggered in the current test
+   environment and completions are compared with <completions>,
+   which should be a space-separated list.  Include a trailing
+   space in <cmdline> when you want to check new argument
+   completion; otherwise, completion is triggered with the
+   context of the last argument.  <message> should state what it
+   is testing.
+
  - test_tick [interval]
 
    The test harness has an internal view of time which is