parser: avoid an unnecessary UPDATE of Person

parser: avoid an unnecessary UPDATE of Person

Analysis of SQL statements showed that when parsing an email, the row
for the Person who sent the email was always getting updated. This is
because the test for updating it only checks if the incoming mail has
*a* name attached to the email address, and not if it has a new name.
Django is not smart enough to figure that out, and so unconditionally
UPDATEs the model when asked to save.

Give it a hand - only update the model and save it if the new name is
in fact different.

Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>