(mu:for-each-message ...) allows an optional expression to be passed
so that only messages matching that expression are searched when
looking for duplicates.
This patch adds a --expr flag so that e.g.
find-dups.scm --expr d:6m..3m
will reduce the range of messages to those between 3 and 6 months ago
to search for duplicates. I found this useful when using expressions
to find messages in a particular year, rather than searching the whole
database. This is often quicker and less worrying than searching the
whole database.
If --expr is not provided, expr should default to #t so that the whole
database is searched as before.
e559b6e732