Prevent a jar from being repackaged more than once

Previously, Repackager would repackage a jar file as many times as
it was asked to do so. This lead to problems if a user made a mistake
when using Maven that led to the package phase being driven twice,
for example by running "mvn clean install package".

This commit updates Repackager so that a repackage call becomes a
no-op if the source jar's manifest already contains the
Spring-Boot-Version attribute which is added by repackaging.

Fixes #1251