add_message: Use sha-1 in place of overly long message ID.

Since Xapian has a limit on the maximum length of a term, we have
to check for that before trying to add the message ID as a term.

This fixes the bug reported by Mike Hommey here:

	<20091120132625.GA19246@glandium.org>

I've also constructed 20 files with a range of message ID lengths
centered around the Xapian term-length limit which I'll use to seed a
new test suite soon.