Rework fixed archive deduplication into new deduplicate action.

As per discussion on the mailing list (http://tech.groups.yahoo.com/group/todotxt/message/3775), the automatic deduplication during archiving is unexpected and difficult to enforce in other implementations. Rather, make this a separate (optional) action.

tests/t1910-deduplicate.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+
+test_description='deduplicate functionality
+
+Ensure we can deduplicate items successfully.
+'
+. ./test-lib.sh
+
+cat >> todo.txt<<EOF
+duplicated
+two
+x done
+duplicated
+double task
+double task
+three
+EOF
+
+test_todo_session 'deduplicate with duplicates' <<EOF
+>>> todo.sh deduplicate
+TODO: 2 duplicate task(s) removed
+
+>>> todo.sh -p ls
+4 double task
+1 duplicated
+5 three
+2 two
+3 x done
+--
+TODO: 5 of 5 tasks shown
+EOF
+
+test_todo_session 'deduplicate without duplicates' <<EOF
+>>> todo.sh deduplicate
+TODO: No duplicate tasks found
+EOF
+
+test_done