Fix to index the "Re" term present in any subject.

This was a misfeature where notmuch had extra code that just threw
away legitimate information. It was never indexing an initial "Re"
term in a subject. But some users have legitimately wanted to search
for this term.

The original code was written this way merely for strict compatiblity
with the indexing performed by sup, but we're not taking advantage of
that now anyway.