Bugfix: depri first checks validity of item argument syntax before searching for todo.

Added testcase for the basic depriority command.

tests/t1700-depri.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+test_description='basic depriority functionality
+'
+. ./test-lib.sh
+
+test_todo_session 'depriority usage' <<EOF
+>>> todo.sh depri B B
+usage: todo.sh depri ITEM#
+=== 1
+EOF
+
+test_todo_session 'depriority nonexistant item' <<EOF
+>>> todo.sh depri 42
+42: No such todo.
+=== 1
+EOF
+
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(A) notice the sunflowers
+stop
+EOF
+test_todo_session 'basic depriority' <<EOF
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+1 (B) smell the uppercase Roses +flowers @outside
+3 stop
+--
+TODO: 3 of 3 tasks shown
+
+>>> todo.sh depri 1
+1: smell the uppercase Roses +flowers @outside
+TODO: 1 deprioritized.
+
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+1 smell the uppercase Roses +flowers @outside
+3 stop
+--
+TODO: 3 of 3 tasks shown
+EOF
+
+test_done