notmuch: fix corrupt viewing of messages

The mutt records offset of email body when parse email header.

Unfortunately when we check notmuch DB changes when we don't ask for
email path in the same way. So it's possible that for emails that are
stored in multiple folders (you sent the same email with the same ID
to multiple mailing lists) we get a different path, then notmuch mutt
code updates the path, but no the offset... result is corrupted body.

Signed-off-by: Karel Zak <kzak@redhat.com>